Summary

    A powerful and robust templating system for Python.


Overview

    EmPy is a system for embedding Python expressions and statements
    in template text; it takes an EmPy source file, processes it, and
    produces output.  This is accomplished via expansions, which are
    special signals to the EmPy system and are set off by a special
    prefix (by default the at sign, '@').  EmPy can expand arbitrary
    Python expressions and statements in this way, as well as a
    variety of special forms.  Textual data not explicitly delimited
    in this way is sent unaffected to the output, allowing Python to
    be used in effect as a markup language.  Also supported are
    callbacks via hooks, recording and playback via diversions, and
    dynamic, chainable filters.  The system is highly configurable via
    command line options and embedded commands.

    Expressions are embedded in text with the '@(...)' notation;
    variations include conditional expressions with '@(...?...!...)'
    and the ability to handle thrown exceptions with '@(...$...)'.  As
    a shortcut, simple variables and expressions can be abbreviated as
    '@variable', '@object.attribute', '@function(arguments)',
    '@sequence' [index], and combinations.  Full-fledged statements
    are embedded with '@{...}'.  Control flow in terms of conditional
    or repeated expansion is available with '@[...]'.  A '@' followed
    by a whitespace character (including a newline) expands to
    nothing, allowing string concatenations and line continuations.
    Comments are indicated with '@#' and consume the rest of the line,
    up to and including the trailing newline.  '@%' indicate
    "significators," which are special forms of variable assignment
    intended to specify per-file identification information in a
    format which is easy to parse externally.  Context name and line
    number changes can be done with '@?' and '@!' respectively.
    '@<...>' markups are customizeable by the user and can be used for
    any desired purpose.  Escape sequences analogous to those in C can
    be specified with '@\...', and finally a '@@' sequence expands to
    a single literal at sign.


Getting the software

    The current version of empy is 3.3.3.

    The latest version of the software is available in a tarball here:
    "http://www.alcyone.com/software/empy/empy-latest.tar.gz",
    http://www.alcyone.com/software/empy/empy-latest.tar.gz.

    The official URL for this Web site is
    "http://www.alcyone.com/software/empy/",
    http://www.alcyone.com/software/empy/.


Requirements

    EmPy should work with any version of Python from 2.4 onward,
    including 3.x.


License

    This code is released under the "LGPL",
    http://www.gnu.org/copyleft/lesser.html.


Mailing lists

    There are two EmPy related mailing lists available.  The first is
    a receive-only, very low volume list for important announcements
    (including releases).  To subscribe, send an email to
    "empy-announce-list-subscribe@alcyone.com",
    mailto:empy-announce-list-subscribe@alcyone.com.

    The second is a general discussion list for topics related to
    EmPy, and is open for everyone to contribute; announcements
    related to EmPy will also be made on this list.  The author of
    EmPy (and any future developers) will also be on the list, so it
    can be used not only to discuss EmPy features with other users,
    but also to ask questions of the author(s).  To subscribe, send an
    email to "empy-list-subscribe@alcyone.com",
    mailto:empy-list-subscribe@alcyone.com.


Basics

    EmPy is intended for embedding Python code in otherwise
    unprocessed text.  Source files are processed, and the results are
    written to an output file.  Normal text is sent to the output
    unchanged, but markups are processed, expanded to their results,
    and then written to the output file as strings (that is, with the
    'str' function, not 'repr').  The act of processing EmPy source
    and handling markups is called "expansion."

    Code that is processed is executed exactly as if it were entered
    into the Python interpreter; that is, it is executed with the
    equivalent of 'eval' (for expressions) and 'exec' (for
    statements).  EmPy is intended to be a very thin (though powerful)
    layer on top of a running Python system; Python and EmPy files can
    be mixed together (via command line options) without
    complications.

    By default the embedding prefix is the at sign ('@'), which
    appears neither in valid Python code nor commonly in arbitrary
    texts; it can be overridden with the -p option (or with the
    'empy.setPrefix' function).  The prefix indicates to the EmPy
    interpreter that a special sequence follows and should be
    processed rather than sent to the output untouched (to indicate a
    literal at sign, it can be doubled as in '@@').

    When the interpreter starts processing its target file, no modules
    are imported by default, save the 'empy' pseudomodule (see below),
    which is placed in the globals; the 'empy' pseudomodule is
    associated with a particular interpreter -- in fact, they are the
    same object -- and it is important that it not be removed from
    that interpreter's globals, nor that it be shared with other
    interpreters running concurrently (a name other than 'empy' can be
    specified with the -m option).  The globals are not cleared or
    reset in any way.  It is perfectly legal to set variables or
    explicitly import modules and then use them in later markups,
    *e.g.*, '@{import time} ... @time.time()'.  Scoping rules are as
    in normal Python, although all defined variables and objects are
    taken to be in the global namespace.

    Indentation is significant in Python, and therefore is also
    significant in EmPy.  EmPy statement markups ('@{...}'), when
    spanning multiple lines, must be flush with the left margin.  This
    is because (multiline) statement markups are not treated specially
    in EmPy and are simply passed to the Python interpreter, where
    indentation is significant.

    Activities you would like to be done before any processing of the
    main EmPy file can be specified with the -I, -D, -E, -F, and -P
    options.  -I imports modules, -D executes a Python variable
    assignment, -E executes an arbitrary Python (not EmPy) statement,
    -F executes a Python (not EmPy) file, and -P processes an EmPy
    (not Python) file.  These operations are done in the order they
    appear on the command line; any number of each (including, of
    course, zero) can be used.


Expansions

    The following markups are supported.  For concreteness below, '@'
    is taken for the sake of argument to be the prefix character,
    although this can be changed.

    **'@# COMMENT NEWLINE'** -- A comment.  Comments, including the
      trailing newline, are stripped out completely.  Comments should
      only be present outside of expansions.  The comment itself is
      not processed in any way: It is completely discarded.  This
      allows '@#' comments to be used to disable markups.  *Note:* As
      special support for "bangpaths" in Unix-like operating systems,
      if the first line of a file (or indeed any context) begins with
      '#!', and the interpreter has a 'processBangpaths' option set to
      true (default), it is treated as a '@#' comment.  A '#!'
      sequence appearing anywhere else will be handled literally and
      unaltered in the expansion.  Example::

          @# This line is a comment.
          @# This will NOT be expanded: @x.

    **'@? NAME NEWLINE'** -- Set the name of the current context to be
      the given string.  Variables are not allowed here; the name is
      treated as a literal.  (If you wish to use arbitrary
      expressions, use the 'empy.setContextName' function instead.)
      Example::

          @?NewName
          The context name is now @empy.identify()[0] (NewName).

    **'@! INTEGER NEWLINE'** -- Set the line number of the current
      context to be the given integer value; this is similar to the
      '#line' C preprocessor directive.  This is done in such a way
      that the *next* line will have the specified numeric value, not
      the current one.  Expressions are not allowed here; the number
      must be a literal integer.  (If you wish to use arbitrary
      expressions, use the 'empy.setContextLine' function instead.)
      Example::

          @!100
          The context line is now @empy.identify()[1] (100).

    **'@ WHITESPACE'** -- A '@' followed by one whitespace character
      (a space, horizontal tab, vertical tab, carriage return, or
      newline) is expanded to nothing; it serves as a way to
      explicitly separate two elements which might otherwise be
      interpreted as being the same symbol (such as '@name@ s' to mean
      '@(name)s' -- see below).  Also, since a newline qualifies as
      whitespace here, the lone '@' at the end of a line represents a
      line continuation, similar to the backslash in other languages.
      Coupled with statement expansion below, spurious newlines can be
      eliminated in statement expansions by use of the '@{...}@'
      construct.  Example::

          This will appear as one word: salt@ water.
          This is a line continuation; @
          this text will appear on the same line.

    **'@\ ESCAPE_CODE'** -- An escape code.  Escape codes in EmPy are
      similar to C-style escape codes, although they all begin with
      the prefix character.  Valid escape codes include:

          '@\0' -- NUL, null

          '@\a' -- BEL, bell

          '@\b' -- BS, backspace

          '@\d' -- three-digital decimal code DDD

          '@\e' -- ESC, escape

          '@\f' -- FF, form feed

          '@\h' -- DEL, delete

          '@\n' -- LF, linefeed character, newline

          '@\oOOO' -- three-digit octal code OOO

          '@\qQQQQ' -- four-digit quaternary code QQQQ

          '@\r' -- CR, carriage return

          '@\s' -- SP, space

          '@\t' -- HT, horizontal tab

          '@\v' -- VT, vertical tab

          '@\xHH' -- two-digit hexadecimal code HH

          '@\z' -- EOT, end of transmission

          '@^X' -- the control character ^X

      Unlike in C-style escape codes, escape codes taking some number
      of digits afterward always take the same number to prevent
      ambiguities.  Furthermore, unknown escape codes are treated as
      parse errors to discourage potential subtle mistakes.  Note
      that, while '@\0' represents the NUL character, to represent an
      octal code, one must use '@\o...', in contrast to C.  Example::

          This embeds a newline.@\nThis is on the following line.
          This beeps!@\a
          There is a tab here:@\tSee?
          This is the character with octal code 141: @\o141.

    **'@@'** -- A literal at sign ('@').  To embed two adjacent at
      signs, use '@@@@', and so on.  Any literal at sign that you wish
      to appear in your text must be written this way, so that it will
      not be processed by the system.  *Note:* If a prefix other than
      '@' has been chosen via the command line option, one expresses
      that literal prefix by doubling it, not by appending a '@'.
      Example::

          The prefix character is @@.
          To get the expansion of x you would write @@x.

    **'@)', '@]', '@}'** -- These expand to literal close parentheses,
      close brackets, and close braces, respectively; these are
      included for completeness and explicitness only.  Example::

          This is a close parenthesis: @).

    **'@"..."', '@"""..."""', etc.** -- These string literals expand
      to the literals themselves, so '@"test"' expands to 'test'.
      Since they are inherently no-operations, the only reason for
      their use is to override their behavior with hooks.

    **'@( EXPRESSION )'** -- Evaluate an expression, and expand with
      the string (via a call to 'str') representation evaluation of
      that expression.  Whitespace immediately inside the parentheses
      is ignored; '@( expression )' is equivalent to '@(expression)'.
      If the expression evaluates to 'None', nothing is expanded in
      its place; this allows function calls that depend on side
      effects (such as printing) to be called as expressions.  (If you
      really *do* want a 'None' to appear in the output, then use the
      Python string '"None"'.)  *Note:* If an expression prints
      something to 'sys.stdout' as a side effect, then that printing
      will be spooled to the output *before* the expression's return
      value is.  Example::

          2 + 2 is @(2 + 2).
          4 squared is @(4**2).
          The value of the variable x is @(x).
          This will be blank: @(None).

    **'@( TEST ? THEN (! ELSE)_opt ($ EXCEPT)_opt )'** -- A special
      form of expression evaluation representing conditional and
      protected evaluation.  Evaluate the "test" expression; if it
      evaluates to true (in the Pythonic sense), then evaluate the
      "then" section as an expression and expand with the 'str' of
      that result.  If false, then the "else" section is evaluated and
      similarly expanded.  The "else" section is optional and, if
      omitted, is equivalent to 'None' (that is, no expansion will
      take place).  *Note*: For backward compatibility, the "else"
      section delimiter, '!', may be expressed as a ':'.  This
      behavior is supported but deprecated.

      If the "except" section is present, then if any of the prior
      expressions raises an exception when evaluated, the expansion
      will be replaced with the evaluation of the except expression.
      (If the "except" expression itself raises, then that exception
      will be propagated normally.)  The except section is optional
      and, if omitted, is equivalent to 'None' (that is, no expansion
      will take place).  An exception (cough) to this is if one of
      these first expressions raises a SyntaxError; in that case the
      protected evaluation lets the error through without evaluating
      the "except" expression.  The intent of this construct is to
      except runtime errors, and if there is actually a syntax error
      in the "try" code, that is a problem that should probably be
      diagnosed rather than hidden.  Example::

          What is x? x is @(x ? "true" ! "false").
          Pluralization: How many words? @x word@(x != 1 ? 's').
          The value of foo is @(foo $ "undefined").
          Division by zero is @(x/0 $ "illegal").

    **'@ SIMPLE_EXPRESSION'** -- As a shortcut for the '@(...)'
      notation, the parentheses can be omitted if it is followed by a
      "simple expression."  A simple expression consists of a name
      followed by a series of function applications, array
      subscriptions, or attribute resolutions, with no intervening
      whitespace.  For example:

          - a name, possibly with qualifying attributes (*e.g.*,
            '@value', '@os.environ').

          - a straightforward function call (*e.g.*, '@min(2, 3)',
            '@time.ctime()'), with no space between the function name
            and the open parenthesis.

          - an array subscription (*e.g.*, '@array[index]',
            '@os.environ[name]', with no space between the name and
            the open bracket.

          - any combination of the above (*e.g.*,
            '@function(args).attr[sub].other[i](foo)').

      In essence, simple expressions are expressions that can be
      written ambiguously from text, without intervening space.  Note
      that trailing dots are not considered part of the expansion
      (*e.g.*, '@x.' is equivalent to '@(x).', not '@(x.)', which
      would be illegal anyway).  Also, whitespace is allowed within
      parentheses or brackets since it is unambiguous, but not between
      identifiers and parentheses, brackets, or dots.  Explicit
      '@(...)' notation can be used instead of the abbreviation when
      concatenation is what one really wants (*e.g.*, '@(word)s' for
      simple pluralization of the contents of the variable 'word').
      As above, if the expression evaluates to the 'None' object,
      nothing is expanded.  Note that since a curly appearing where
      EmPy would expect an open parenthesis or bracket in is
      meaningless in Python, it is treated as a parse error (*e.g.*,
      '@x{1, 2}' results in an error).  Example::

          The value of x is @x.
          The ith value of a is @a[i].
          The result of calling f with q is @f(q).
          The attribute a of x is @x.a.
          The current time is @time.ctime(time.time()).
          The current year is @time.localtime(time.time())[0].
          These are the same: @min(2,3) and @min(2, 3).
          But these are not the same: @min(2, 3) vs. @min (2, 3).
          The plural of @name is @(name)s, or @name@ s.

    **'@` EXPRESSION `'** -- Evaluate a expression, and expand with
      the 'repr' (instead of the 'str' which is the default) of the
      evaluation of that expression.  This expansion is primarily
      intended for debugging and is unlikely to be useful in actual
      practice.  That is, a '@`...`' is identical to '@(repr(...))'.
      Example::

          The repr of the value of x is @`x`.
          This print the Python repr of a module: @`time`.
          This actually does print None: @`None`.

    **'@: EXPRESSION : DUMMY :'** -- Evaluate an expression and then
      expand to a '@:', the original expression, a ':', the evaluation
      of the expression, and then a ':'.  The current contents of the
      dummy area are ignored in the new expansion.  In this sense it
      is self-evaluating; the syntax is available for use in
      situations where the same text will be sent through the EmPy
      processor multiple times.  Example::

          This construct allows self-evaluation:
          @:2 + 2:this will get replaced with 4:

    **'@{ STATEMENTS }'** -- Execute a (potentially compound)
      statement; statements have no return value, so the expansion is
      not replaced with anything.  Multiple statements can either be
      separated on different lines, or with semicolons; indentation is
      significant, just as in normal Python code.  Statements,
      however, can have side effects, including printing; output to
      'sys.stdout' (explicitly or via a 'print' statement) is
      collected by the interpreter and sent to the output (unless this
      behavior is suppressed with the -n option).  The usual Python
      indentation rules must be followed, although if the statement
      consists of only one statement, leading and trailing whitespace
      is ignored (*e.g.*, '@{ print time.time() }' is equivalent to
      '@{print time.time()}').  Example::

          @{x = 123}
          @{a = 1; b = 2}
          @{print time.time()}
          @# Note that extra newlines will appear above because of the
          @# newlines trailing the close braces.  To suppress them
          @# use a @ before the newline:
          @{
          for i in range(10):
              print "i is %d" % i
          }@
          @{print "Welcome to EmPy."}@

    **'@% KEY (WHITESPACE VALUE)_opt NEWLINE'** -- Declare a
      significator.  Significators consume the whole line (including
      the trailing newline), and consist of a key string containing no
      whitespace, and than optional value prefixed by whitespace.  The
      key may not start with or contain internal whitespace, but the
      value may; preceding or following whitespace in the value is
      stripped.  Significators are totally optional, and are intended
      to be used for easy external (that is, outside of EmPy)
      identification when used in large scale environments with many
      EmPy files to be processed.  The purpose of significators is to
      provide identification information about each file in a special,
      easy-to-parse form so that external programs can process the
      significators and build databases, independently of EmPy.
      Inside of EmPy, when a significator is encountered, its key,
      value pair is translated into a simple assignment of the form
      '__KEY__ = VALUE' , where "__KEY__" is the key string with two
      underscores on either side and "VALUE" is a Python expression.
      Example::

          @%title     "Gravitation"
          @%author    "Misner", "Thorne", "Wheeler"
          @%publisher "W.H. Freeman and Company"
          @%pages     1279
          @%keywords  'physics', 'gravity', 'Einstein', 'relativity'
          @%copyright 1970, 1971

    **'@< CONTENTS >'** -- Invoke a custom markup.  The custom markup
      is a special markup reserved for use by the user; it has no
      prescribed meaning on its own.  If 'contents' is a string
      representing what appears in between the angle brackets, then
      expanding this markup is equivalent to
      'empy.invokeCallback(contents)'.  See the "Custom markup"
      section for more information.


Control

    EmPy version 3 and above includes the ability to direct
    conditional and repeated expansion of blocks of EmPy code with
    control markups (the obsolescent "substitution" markups are
    unavailable as of version 3.0).  Control markups have analogs to
    control flow structures in Python such as 'if/elif/else', 'for', and
    'while'.  Control markups are set off with the '@[...]' notation.

    Control markups are designed to be used in precisely the same way
    that their internal Python analogues are used, except that the
    control markups are intended to be used where there is much more
    markup than control structure.

    Some control markups are considered "primary," (*e.g.*, 'if',
    'for', 'while') as they begin a control markup.  Others are
    considered "secondary," since they can only appear inside control
    flow markups delineated by primary markups (*e.g.*, 'elif',
    'else', 'continue', 'break').

    Since EmPy, unlike Python, cannot use indentation to determine
    where control structures begin and end, all primary control
    markups *must* be followed by a corresponding terminating control
    markup::

        @[PRIMARY ...]...@[end PRIMARY]

    (where 'PRIMARY' represents one of the primary keywords).  The end
    markup is mandatory, as is the space between the 'end' and the
    starting keyword.  For instance::

        @# If `person' is alive, show their age.
        @person.name is @
        @[if person.isAlive]@person.age@[else]dead@[end if].

    All primary markups must be terminated in this way, and the
    keyword appearing in the appropriate 'end' markup must match the
    primary markup it corresponds to; if either of these conditions
    are not satisfied, the result is a parse error.  Everything
    between the starting control flow marker ('@[PRIMARY ...]') and
    the ending marker ('@[end PRIMARY]') -- including other markups,
    even control markups -- is considered part of the markup.  Control
    markups can be nested::

        @# Print all non-false elements on separate lines.
        @[for elem in elements]@[if elem]@elem@\n@[end if]@[end for]

    Three major types of primary control markups are available:
    conditional (*e.g.*, 'if', 'try'), looping (*e.g.*, 'for',
    'while'), and definitional (*e.g.*, 'def', discussed below).
    Conditional control markups conditionally expand their contents,
    whereas looping control markups repeatedly expand their contents.
    The third type, definitional markups, will define new objects in
    the globals relating to their contents.  Conditional and looping
    markups also differ in one substantial respect: Looping constructs
    support '@[continue]' and '@[break]' markups which, like their
    Python equivalents, continue with the next iteration or break out
    of the innermost looping construct, respectively ('@[continue]'
    and '@[break]' markups have no meaning inside conditional markups
    and are an error).  Also like their Python equivalents,
    '@[continue]' and '@[break]' may appear inside nested markups, so
    long as they ultimately are contained by at least one looping
    control markup::

        @# Walk a long a linked list, printing each element.
        @[while 1]@
        @node
        @{node = node.next}@
        @[if not node]@[break]@[end if]@
        @[end while]

    The provided markups are designed to mimic the internal Python
    control structures as closely as possible.  The supported control
    markups are (the phrases in all uppercase are intended to signify
    user-selectable patterns)::

	@[if CONDITION1]...@[elif CONDITION2]...@[else]...@[end if]
	@[try]...@[except ...]...@[except ...]...@[end try]
	@[try]...@[finally]...@[end try]
	@[for VARIABLE in SEQUENCE]...@[else]...@[end for]
	@[while CONDITION]...@[else]...@[end while]
        @[def SIGNATURE]...@[end def]

    All recognizable forms behave like their Python equivalents; 'if'
    can contain multiple 'elif' secondary markups within it; the
    'else' markups are optional (but must appear at the end), the
    'try' form with the 'except' clause can contain multiple ones
    which are handled in sequence, the 'try' form can either contain
    one or more 'except' clauses or one 'finally' clause (but not
    both), and the 'for' and 'while' structures can contain 'continue'
    or 'break' clauses internally (even if contained within other
    markups).

    The third type of primary control markup is "definitional," in
    that they create objects in the globals for later use (*e.g.*,
    'def').  This allows the definition of a callable object which,
    when called, will expand the contained markup (which can in turn,
    of course, contain further markups).  The argument to the markup
    can be any legal Python function signature::

        @[def f(x, y, z=2, *args, **keywords)]...@[end def]

    would define a function in the globals named 'f' that takes the
    given arguments.  A macro markup of the form '@[def
    SIGNATURE]CODE@[end def]' is equivalent to the Python code::

        def SIGNATURE:
            r"""CODE""" # so it is a doc string
            empy.expand(r"""CODE""", locals())

    That is, it creates a Python function with the same name and
    function arguments, whose docstring is the contents of the EmPy
    markup that will be expanded when called.  And, when called, it
    will expand those contents, with the locals passed in.


Unicode support

    EmPy version 3.1 and above includes intrinsic Unicode support.
    EmPy's Unicode support defers to Python's internal Unicode
    support, available in Python 2.0 and up, in order to allow
    seamless and transparent translation of different encodings to the
    native Python Unicode format.

    Knowledge of Python's Unicode support is expected, although not
    completely required, to gain full benefit of EmPy's Unicode
    features.  To enable Unicode support, start EmPy with the
    -u/--unicode option.  EmPy will then transparently encode from the
    input stream, process markups internally with native Unicode, and
    then decode transparently to the output stream.

    By default, Python sets 'sys.stdin' and 'sys.stdout' with a
    default encoding which is accessible via
    'sys.getdefaultencoding()'; encodings are represented by string
    names.  These streams have encodings set by the system and
    *cannot* be changed.

    However, encodings for newly created files (files to be read when
    specified on the command line, and/or files to be written when
    used with the -o and -a arguments) can be specified for EmPy via
    command line options.  The --unicode-encoding option
    simultaneously indicates the encoding to be used for both input
    and output, whereas the --unicode-input-encoding and
    --unicode-output-encoding options can each be used to specify
    different encodings for both input and output.  (If an encoding is
    not explicitly indicated, it resorts to the system default in
    'sys.getdefaultencoding()', which is locale dependent.)

    Python's Unicode implementation has the concept of error handlers,
    registered with the 'codecs' module, which can be specified to
    determine what action should take place if input cannot be decoded
    into Unicode, or Unicode cannot be encoded into output.  EmPy uses
    these same "errors," as they are called, and can be specified via
    command line options.  The three most common error handlers are:
    'ignore', where invalid sequences are simply ignored; 'replace',
    where invalid sequences are replaced with an encoding-specific
    indicator, usually a question mark; and 'strict', where invalid
    sequences raise an error.  The --unicode-errors command line
    option specifies the same error handler to be used for both input
    and output, and the --unicode-input-errors and
    --unicode-output-errors options can specify different error
    handlers for input and output.  If an error handler is not
    explicitly specified, the 'strict' handler (which will raise
    errors) is used.

    Remember, to specify input encodings or errors that will take
    effect, one cannot take input from 'sys.stdin' and must explicitly
    specify an EmPy file to process on the command line.  Similarly,
    for output encodings or errors, 'sys.stdout' cannot be used and an
    explicit output file must be specified with the -o or -a options.
    It is perfectly valid to enable the Unicode subsystem (-u option)
    while using 'sys.stdin' and 'sys.stdout', but the encodings and
    errors of these preexisting streams cannot be changed.

    Combined with the --no-prefix option, which disables all markup
    processing, EmPy can act merely as an encoding translator, relying
    on Python's Unicode facilities::

        em.py --no-prefix \
            --unicode-input-encoding=utf-8 \
            --unicode-output-encoding=latin-1 \
            -o filename.Latin-1 filename.UTF-8


Significators

    Significators, introduced in EmPy version 1.2, are intended to
    represent special assignment in a form that is easy to externally
    parse.  For instance, if one has a system that contains many EmPy
    files, each of which has its own title, one could use a 'title'
    significator in each file and use a simple regular expression to
    find this significator in each file and organize a database of the
    EmPy files to be built.  This is an easier proposition than, for
    instance, attempting to grep for a normal Python assignment
    (inside a '@{...}' expansion) of the desired variable.

    Significators look like the following::

        @%KEY VALUE

    including the trailing newline, where "key" is a name and "value"
    is a Python expression, and are separated by any whitespace.  This
    is equivalent to the following Python code::

        __KEY__ = VALUE

    That is to say, a significator key translates to a Python variable
    consisting of that key surrounded by double underscores on either
    side.  The value may contain spaces, but the key may not.  So::

        @%title "All Roads Lead to Rome"

    translates to the Python code::

        __title__ = "All Roads Lead to Rome"

    but obviously in a way that easier to detect externally than if
    this Python code were to appear somewhere in an expansion.  Since
    significator keys are surrounded by double underscores,
    significator keys can be any sequence of alphanumeric and
    underscore characters; choosing '123' is perfectly valid for a
    significator (although straight), since it maps to the name
    '__123__' which is a legal Python identifier.

    Note the value can be any Python expression.  The value can be
    omitted; if missing, it is treated as 'None'.

    Significators are completely optional; it is completely legal for
    a EmPy file or files to be processed without containing any
    significators.  Significators can appear anywhere within a file
    outside of other markups, but typically they are placed near the
    top of the file to make them easy to spot and edit by humans.

    A regular expression string designed to match significators (with
    the default prefix) is available as 'empy.SIGNIFICATOR_RE_STRING',
    and also is a toplevel definition in the 'em' module itself.



Diversions

    EmPy supports an extended form of diversions, which are a
    mechanism for deferring and recalling output on demand, similar to
    the functionality included in m4.  Multiple "streams" of output
    can be diverted (deferred) and undiverted (recalled) in this
    manner.  A diversion is identified with a name, which is any
    immutable object such an integer or string.  When recalled,
    diverted code is *not* resent through the EmPy interpreter
    (although a filter could be set up to do this).

    By default, no diversions take place.  When no diversion is in
    effect, processing output goes directly to the specified output
    file.  This state can be explicitly requested at any time by
    calling the 'empy.stopDiverting' function.  It is always legal to
    call this function.

    When diverted, however, output goes to a deferred location which
    can then be recalled later.  Output is diverted with the
    'empy.startDiversion' function, which takes an argument that is
    the name of the diversion.  If there is no diversion by that name,
    a new diversion is created and output will be sent to that
    diversion; if the diversion already exists, output will be
    appended to that preexisting diversion.

    Output send to diversions can be recalled in two ways.  The first
    is through the 'empy.playDiversion' function, which takes the
    name of the diversion as an argument.  This recalls the named
    diversion, sends it to the output, and then erases that
    diversion.  A variant of this behavior is the
    'empy.replayDiversion', which recalls the named diversion but does
    not eliminate it afterwards; 'empy.replayDiversion' can be
    repeatedly called with the same diversion name, and will replay
    that diversion repeatedly.  'empy.createDiversion' create a
    diversion without actually diverting to it, for cases where you
    want to make sure a diversion exists but do not yet want to send
    anything to it.

    The diversion object itself can be retrieved with
    'empy.retrieveDiversion'.  Diversions act as writable
    file-objects, supporting the usual 'write', 'writelines', 'flush',
    and 'close' methods.  The data that has been diverted to them can
    be retrieved in one of two ways; either through the 'asString'
    method, which returns the entire contents of the diversion as a
    single strong, or through the 'asFile' method, which returns the
    contents of the diversion as a readable (not writable) file-like
    object.

    Diversions can also be explicitly deleted without recalling them
    with the 'empy.purgeDiversion' function, which takes the desired
    diversion name as an argument.

    Additionally there are three functions which will apply the above
    operations to all existing diversions: 'empy.playAllDiversions',
    'empy.replayAllDiversions', and 'empy.purgeAllDiversions'.  All
    three will do the equivalent of a 'empy.stopDiverting' call before
    they do their thing.

    The name of the current diversion can be requested with the
    'empy.getCurrentDiversion' function; also, the names of all
    existing diversions (in sorted order) can be retrieved with
    'empy.getAllDiversions'.

    When all processing is finished, the equivalent of a call to
    'empy.playAllDiversions' is done.


Filters

    EmPy also supports dynamic filters, introduced in version 1.3.
    Filters are put in place right "before" the final output file, and
    so are only invoked after all other processing has taken place
    (including interpreting and diverting).  Filters take input, remap
    it, and then send it to the output.

    The current filter can be retrieved with the 'empy.getFilter'
    function.  The filter can be cleared (reset to no filter) with
    'empy.resetFilter' and a special "null filter" which does not send
    any output at all can be installed with 'empy.nullFilter'.  A
    custom filter can be set with the 'empy.setFilter' function; for
    convenience, specialized shortcuts for filters preexist and can be
    used in lieu of actual 'empy.Filter' instances for the
    'empy.setFilter' or 'empy.attachFilter' argument:

    - 'None' is a special filter meaning "no filter"; when installed,
      no filtering whatsoever will take place.  'empy.setFilter(None)'
      is equivalent to 'empy.resetFilter()'.

    - '0' (or any other numeric constant equal to zero) is another
      special filter that represents the null filter; when installed,
      no output will ever be sent to the filter's sink.

    - A filter specified as a function (or lambda) is expected to take
      one string argument and return one string argument; this filter
      will execute the function on any input and use the return value
      as output.

    - A filter that is a string is a 256-character table is
      substituted with the result of a call to 'string.translate'
      using that table.

    - A filter can be an instance of a subclass of 'empy.Filter'.
      This is the most general form of filter.  (In actuality, it can
      be any object that exhibits a 'Filter' interface, which would
      include the normal file-like 'write', 'flush', and 'close'
      methods, as well as 'next', 'attach', and 'detach' methods for
      filter-specific behavior.)

    - Finally, the argument to 'empy.setFilter' can be a Python list
      consisting of one or more of the above objects.  In that case,
      those filters are chained together in the order they appear in
      the list.  An empty list is the equivalent of 'None'; all
      filters will be uninstalled.

    Filters are, at their core, simply file-like objects (minimally
    supporting 'write', 'flush', and 'close' methods that behave in
    the usual way) which, after performing whatever processing they
    need to do, send their work to the next file-like object or filter
    in line, called that filter's "sink."  That is to say, filters can
    be "chained" together; the action of each filter takes place in
    sequence, with the output of one filter being the input of the
    next.  Additionally, filters support a '_flush' method (note the
    leading underscore) which will always flush the filter's
    underlying sink; this method should be not overridden.

    Filters also support three additional methods, not part of the
    traditional file interface: 'attach', which takes as an argument a
    file-like object (perhaps another filter) and sets that as the
    filter's "sink" -- that is, the next filter/file-like object in
    line.  'detach' (which takes no arguments) is another method which
    flushes the filter and removes its sink, leaving it isolated.
    Finally, 'next' is an accessor method which returns the filter's
    sink -- or 'None', if the filter does not yet have a sink
    attached.

    To create your own filter, you can create an object which supports
    the above described interface, or simply derive from the
    'empy.Filter' class and override its 'write' and possibly 'flush'
    methods.  You can chain filters together by passing them as
    elements in a list to the 'empy.setFilter' function, or you can
    chain them together manually with the 'attach' method::

        firstFilter.attach(secondFilter)
        empy.setFilter(firstFilter)

    or just let EmPy do the chaining for you::

        empy.setFilter([firstFilter, secondFilter])

    In either case, EmPy will walk the filter chain and find the end
    and then hook that into the appropriate interpreter stream; you
    need not do this manually.  The function 'empy.attachFilter' can
    be used to attach a single filter (or shortcut, as above) to the
    end of a currently existing chain.  Note that unlike its cousin
    'empy.setFilter', one cannot pass a sequence of filters (or filter
    shortcuts) to 'empy.attachFilter'.  (If there is no existing
    filter chain installed, 'empy.attachFilter' will behave the same
    as 'empy.setFilter'.)

    Subclasses of 'empy.Filter' are already provided with the above
    null, function, and string functionality described above; they are
    'NullFilter', 'FunctionFilter', and 'StringFilter', respectively.
    In addition, a filter which supports buffering, 'BufferedFilter',
    is provided.  Several variants are included: 'SizeBufferedFilter',
    a filter which buffers into fixed-sized chunks,
    'LineBufferedFilter', a filter which buffers by lines, and
    'MaximallyBufferedFilter', a filter which completely buffers its
    input.


Hooks

    The EmPy system allows for the registry of hooks with a running
    EmPy interpreter.  Originally introduced in version 2.0 and much
    improved in 3.2, hooks are objects, registered with an
    interpreter, whose methods represent specific callbacks.  Any
    number of hook objects can be registered with an interpreter, and
    when a callback is invoked, the associated method on each one of
    those hook objects will be called by the interpreter in sequence.

    Hooks are simply instances, nominally derived from the 'empy.Hook'
    class.  The 'empy.Hook' class itself defines a series of methods,
    with the expected arguments, which would be called by a running
    EmPy interpreter.  This scenario, much improved from the prior
    implementation in 2.0, allows hooks to keep state and have more
    direct access to the interpreter they are running in (the
    'empy.Hook' instance contains an 'interpreter' attribute).

    To use a hook, derive a class from 'empy.Hook' and override the
    desired methods (with the same signatures as they appear in the
    base class).  Create an instance of that subclass, and then
    register it with a running interpreter with the 'empy.addHook'
    function.  (This same hook instance can be removed with the
    'empy.removeHook' function.)

    More than one hook instance can be registered with an interpreter;
    in such a case, the appropriate methods are invoked on each
    instance in the order in which they were registered.  To adjust
    this behavior, an optional 'prepend' argument to the
    'empy.addHook' function can be used dictate that the new hook
    should placed at the *beginning* of the sequence of hooks, rather
    than at the end (which is the default).

    All hooks can be enabled and disabled entirely for a given
    interpreter; this is done with the 'empy.enableHooks' and
    'empy.disableHooks' functions.  By default hooks are enabled, but
    obviously if no hooks have been registered no hook callbacks will
    be made.  Whether hooks are enabled or disabled can be determined
    by calling 'empy.areHooksEnabled'.  To get a (copy of) the list of
    registered hooks, call 'empy.getHooks'.  Finally, to invoke a hook
    manually, use 'empy.invokeHook'.

    For a list of supported hook callbacks, see the 'empy.Hook' class
    definition.

    As a practical example, this sample Python code would print a
    pound sign followed by the name of every file that is included
    with 'empy.include'::

        class IncludeHook(empy.Hook):
            def beforeInclude(self, name, file, locals):
                print "# %s" % name

        empy.addHook(IncludeHook())


Custom markup

    Since version 3.2.1, the markup '@<...>' is reserved for
    user-defined use.  Unlike the other markups, this markup has no
    specified meaning on its own, and can be provided a meaning by the
    user.  This meaning is provided with the use of a "custom
    callback," or just "callback," which can be set, queried, or reset
    using the pseudomodule function.

    The custom callback is a callable object which, when invoked, is
    passed a single argument: a string representing the contents of
    what was found inside the custom markup '@<...>'.

    To register a callback, call 'empy.registerCallback'.  To remove
    one, call 'empy.deregisterCallback'.  To retrieve the callback (if
    any) registered with the interpreter, use 'empy.getCallback'.
    Finally, to invoke the callback just as if the custom markup were
    encountered, call 'empy.invokeCallback'.  For instance, '@<This
    text>' would be equivalent to the call '@empy.invokeCallback("This
    text")'.

    By default, to invoke a callback (either explicitly with
    'empy.invokeCallback' or by processing a '@<...>' custom markup)
    when no callback has been registered is an error.  This behavior
    can be changed with the 'CALLBACK_OPT' option, or the
    --no-callback-error command line option.


Pseudomodule

    The 'empy' pseudomodule is available only in an operating EmPy
    system.  (The name of the module, by default 'empy', can be
    changed with the -m option or the 'EMPY_PSEUDO' environment
    variable).  It is called a pseudomodule because it is not actually
    a module, but rather exports a module-like interface.  In fact,
    the pseudmodule is actually the same internal object as the
    interpreter itself.

    The pseudomodule contains the following functions and objects (and
    their signatures, with a suffixed 'opt' indicating an optional
    argument):

    First, basic identification:

    **'VERSION'** -- A constant variable which contains a
      string representation of the EmPy version.

    **'SIGNIFICATOR_RE_STRING'** -- A constant variable representing a
      regular expression string (using the default prefix) that can be
      used to find significators in EmPy code.

    **'SIGNIFICATOR_RE_SUFFIX'** -- The portion of the significator
      regular expression string excluding the prefix, so that those
      using non-standard prefix can build their own custom regular
      expression string with 'myPrefix + empy.SIGNIFICATOR_RE_SUFFIX'.

    **'interpreter'** -- The instance of the interpreter that is
      currently being used to perform execution.  *Note:* This is now
      obsolete; the pseudomodule is itself the interpreter.  Instead
      of using 'empy.interpreter', simply use 'empy'.

    **'argv'** -- A list consisting of the name of the primary EmPy
      script and its command line arguments, in analogue to the
      'sys.argv' list.

    **'args'** -- A list of the command line arguments following the
      primary EmPy script; this is equivalent to 'empy.argv[1:]'.

    **'identify() -> string, integer'** -- Retrieve identification
      information about the current parsing context.  Returns a
      2-tuple consisting of a filename and a line number; if the file
      is something other than from a physical file (*e.g.*, an
      explicit expansion with 'empy.expand', a file-like object within
      Python, or via the -E or -F command line options), a string
      representation is presented surrounded by angle brackets.  Note
      that the context only applies to the *EmPy* context, not the
      Python context.

    **'atExit(callable)'** -- Register a callable object (such as a
      function) taking no arguments which will be called at the end of
      a normal shutdown.  Callable objects registered in this way are
      called in the reverse order in which they are added, so the
      first callable registered with 'empy.atExit' is the last one to
      be called.  Note that although the functionality is related to
      hooks, 'empy.atExit' does no work via the hook mechanism, and
      you are guaranteed that the interpreter and stdout will be in a
      consistent state when the callable is invoked.

    Context manipulation:

    **'pushContext(name_opt, line_opt)'** -- Create a new context with
      the given name and line and push it on the stack.

    **'popContext()'** -- Pop the top context and dispose of it.

    **'setContextName(name)'** -- Manually set the name of the current
      context.

    **'setContextLine(line)'** -- Manually set the line number of the
      current context; line must be a numeric value.  Note that
      afterward the line number will increment by one for each newline
      that is encountered, as before.

    Globals manipulation:

    **'getGlobals()'** -- Retrieve the globals dictionary for this
      interpreter.  Unlike when calling 'globals()' in Python, this
      dictionary *can* be manipulated and you *can* expect changes you
      make to it to be reflected in the interpreter that holds it.

    **'setGlobals(globals)'** -- Reseat the globals dictionary
      associated with this interpreter to the provided mapping type.

    **'updateGlobals(globals)'** -- Merge the given dictionary into
      this interpreter's globals.

    **'clearGlobals(globals_opt)'** -- Clear out the globals
      (restoring, of course, the 'empy' pseudomodule).  Optionally,
      instead of starting with a refresh dictionary, use the
      dictionary provided.

    **'saveGlobals(deep=True)'** -- Save a copy of the globals onto an
      internal history stack from which it can be restored later.  The
      optional 'deep' argument indicates whether or not the copying
      should be a deep copy (default) or a shallow one.  Copying is
      done with 'copy.deepcopy' or 'copy.copy', respectively.

    **'restoreGlobals(destructive=True)'** -- Restore the most
      recently saved globals from the history stack to as the current
      globals for this instance.  The optional 'destructive' argument
      indicates whether or not the restore should remove the restored
      globals from the history stack (default), or whether it should
      be left there for subsequent restores.

    Types:

    **'Interpreter'** -- The actual interpreter class.

    The following functions allow direct execution; optional 'locals'
    arguments, if specified, are treated as the locals dictionary in
    evaluation and execution:

    **'defined(name, locals_opt)'** -- Return true if the given name
      is defined either in the (optional) locals or the interpreter
      globals; return false otherwise.

    **'evaluate(expression, locals_opt)'** -- Evaluate the given
      expression.

    **'serialize(expression, locals_opt)'** -- Serialize the
      expression, just as the interpreter would:  If it is not None,
      convert it to a string with the 'str' builtin function, and then
      write out the result.  If it evaluates to None, do nothing.

    **'execute(statements, locals_opt)'** -- Execute the given
      statement(s).

    **'single(source, locals_opt)'** -- Interpret the "single" source
      code, just as the Python interactive interpreter would.

    **'import_(name, locals_opt)'** -- Import a module.

    **'atomic(name, value, locals_opt)'** -- Perform a single, atomic
      assignment.  In this case name is the string denoating the name
      of the (single) variable to be assigned to, and value is a
      Python object which the name is to be bound to.

    **'assign(name, value, locals_opt)'** -- Perform general
      assignment.  This decays to atomic assignment (above) in the
      normal case, but supports "tuple unpacking" in the sense that if
      name string contains commas, it is treated as a sequence of
      names and memberwise assignment with each member of the value
      (still a Python object, but which must be a sequence).  This
      function will raise a 'TypeError' or 'ValueError' just like
      Python would if tuple unpacking is not possible (that is, if the
      value is not a sequence or is of an incompatible length,
      respectively).  This only supports the assignment of Python
      identifiers, not arbitrary Python lvalues.

    **'significate(key, value_opt, locals_opt)'** -- Do a manual
      signification.  If 'value' is not specified, it is treated as
      'None'.

    The following functions relate to source manipulation:

    **'include(file_or_filename, locals_opt)'** -- Include another
      EmPy file, by processing it in place.  The argument can either
      be a filename (which is then opened with 'open' in text mode) or
      a file object, which is used as is.  Once the included file is
      processed, processing of the current file continues.  Includes
      can be nested.  The call also takes an optional locals
      dictionary which will be passed into the evaluation function.

    **'expand(string, locals_opt)' -> string** -- Explicitly invoke
      the EmPy parsing system to process the given string and return
      its expansion.  This allows multiple levels of expansion,
      *e.g.*, '@(empy.expand("@(2 + 2)"))'.  The call also takes an
      optional locals dictionary which will be passed into the
      evaluation function.  This is necessary when text is being
      expanded inside a function definition and it is desired that the
      function arguments (or just plain local variables) are available
      to be referenced within the expansion.

    **'quote(string) -> string'** -- The inverse process of
      'empy.expand', this will take a string and return a new string
      that, when expanded, would expand to the original string.  In
      practice, this means that appearances of the prefix character
      are doubled, except when they appear inside a string literal.

    **'escape(string, more_opt) -> string'** -- Given a string, quote
      the nonprintable characters contained within it with EmPy
      escapes.  The optional 'more' argument specifies additional
      characters that should be escaped.

    **'flush()'** -- Do an explicit flush on the underlying stream.

    **'string(string, name_opt, locals_opt)'** -- Explicitly process a
      string-like object.  This differs from 'empy.expand' in that the
      string is directly processed into the EmPy system, rather than
      being evaluated in an isolated context and then returned as a
      string.

    Changing the behavior of the pseudomodule itself:

    **'flatten(keys_opt)'** -- Perform the equivalent of 'from empy
      import ...' in code (which is not directly possible because
      'empy' is a pseudomodule).  If keys is omitted, it is taken as
      being everything in the 'empy' pseudomodule.  Each of the
      elements of this pseudomodule is flattened into the globals
      namespace; after a call to 'empy.flatten', they can be referred
      to simple as globals, *e.g.*, '@divert(3)' instead of
      '@empy.divert(3)'.  If any preexisting variables are bound to
      these names, they are silently overridden.  Doing this is
      tantamount to declaring an 'from ... import ...' which is often
      considered bad form in Python.

    Prefix-related functions:

    **'getPrefix() -> char'** -- Return the current prefix.

    **'setPrefix(char)'** -- Set a new prefix.  Immediately after this
      call finishes, the prefix will be changed.  Changing the prefix
      affects only the current interpreter; any other created
      interpreters are unaffected.  Setting the prefix to None or the
      null string means that no further markups will be processed,
      equivalent to specifying the --no-prefix command line argument.

    Diversions:

    **'stopDiverting()'** -- Any diversions that are currently taking
      place are stopped; thereafter, output will go directly to the
      output file as normal.  It is never illegal to call this
      function.

    **'createDiversion(name)'** -- Create a diversion, but do not
      begin diverting to it.  This is the equivalent of starting a
      diversion and then immediately stopping diversion; it is used in
      cases where you want to make sure that a diversion will exist
      for future replaying but may be empty.

    **'startDiversion(name)'** -- Start diverting to the specified
      diversion name.  If such a diversion does not already exist, it
      is created; if it does, then additional material will be
      appended to the preexisting diversions.

    **'playDiversion(name)'** -- Recall the specified diversion and
      then purge it.  The provided diversion name must exist.

    **'replayDiversion(name)'** -- Recall the specified diversion
      without purging it.  The provided diversion name must exist.

    **'purgeDiversion(name)'** -- Purge the specified diversion
      without recalling it.  The provided diversion name must exist.

    **'playAllDiversions()'** -- Play (and purge) all existing
      diversions in the sorted order of their names.  This call does
      an implicit 'empy.stopDiverting' before executing.

    **'replayAllDiversions()'** -- Replay (without purging) all
      existing diversions in the sorted order of their names.  This
      call does an implicit 'empy.stopDiverting' before executing.

    **'purgeAllDiversions()'** -- Purge all existing diversions
      without recalling them.  This call does an implicit
      'empy.stopDiverting' before executing.

    **'getCurrentDiversion() -> diversion'** -- Return the name of the
      current diversion.

    **'getAllDiversions() -> sequence'** -- Return a sorted list of
      all existing diversions.

    Filters:

    **'getFilter() -> filter'** -- Retrieve the current filter.
      'None' indicates no filter is installed.

    **'resetFilter()'** -- Reset the filter so that no filtering is
      done.

    **'nullFilter()'** -- Install a special null filter, one which
      consumes all text and never sends any text to the output.

    **'setFilter(shortcut)'** -- Install a new filter.  A filter is
      'None' or an empty sequence representing no filter, or '0' for a
      null filter, a function for a function filter, a string for a
      string filter, or an instance of 'empy.Filter' (or a workalike
      object).  If filter is a list of the above things, they will be
      chained together manually; if it is only one, it will be
      presumed to be solitary or to have already been manually chained
      together.  See the "Filters" section for more information.

    **'attachFilter(shortcut)'** -- Attach a single filter (sequences
      are not allowed here) to the end of a currently existing filter
      chain, or if there is no current chain, install it as
      'empy.setFilter' would.  As with 'empy.setFilter', the shortcut
      versions of filters are also allowed here.

    Hooks:

    **'areHooksEnabled()'** -- Return whether or not hooks are
      presently enabled.

    **'enableHooks()'** -- Enable invocation of hooks.  By default
      hooks are enabled.

    **'disableHooks()'** -- Disable invocation of hooks.  Hooks can
      still be added, removed, and queried, but invocation of hooks
      will not occur (even explicit invocation with
      'empy.invokeHook').

    **'getHooks()'** -- Get a (copy of the) list of the hooks
      currently registered.

    **'clearHooks()'** -- Clear all the hooks registered with this
      interpreter.

    **'addHook(hook, prepend_opt)'** -- Add this hook to the hooks
      associated with this interpreter.  By default, the hook is
      appended to the end of the existing hooks, if any; if the
      optional insert argument is present and true, it will be
      prepended to the list instead.

    **'removeHook(hook)'** -- Remove this hook from the hooks
      associated with this interpreter.

    **'invokeHook(_name, ...)'** -- Manually invoke a hook method.
      The remaining arguments are treated as keyword arguments and the
      resulting dictionary is passed in as the second argument to the
      hooks.

    Custom markup callback:

    **'getCallback() -> callback'** -- Retrieve the current callback
      associated with this interpreter, or 'None' if it does not yet
      have one.

    **'registerCallback(callback)'** -- Register a callback to be
      called whenever a custom markup ('@<...>') is encountered.  When
      encountered, 'invokeCallback' is called.

    **'deregisterCallback()'** -- Clear any callback previously
      registered with the interpreter for being called when a custom
      markup is encountered.

    **'invokeCallback(contents)'** -- Invoke a custom callback.  This
      function is called whenever a custom markup ('@<...>') is
      encountered.  It in turn calls the registered callback, with a
      single argument, 'contents', which is a string representing of
      the contents of the custom markup.


Invocation

    Basic invocation involves running the interpreter on an EmPy file
    and some optional arguments.  If no file are specified, or the
    file is named '-', EmPy takes its input from stdin.  One can
    suppress option evaluation (to, say, specify a file that begins
    with a dash) by using the canonical '--' option.

    **'-h'/'--help'** -- Print usage and exit.

    **'-H'/'--extended-help'** -- Print extended usage and exit.
      Extended usage includes a rundown of all the legal expansions,
      escape sequences, pseudomodule contents, used hooks, and
      supported environment variables.

    **'-v'/'--verbose'** -- The EmPy system will print all manner of
      details about what it is doing and what it is processing to
      stderr.

    **'-V'/'--version'** -- Print version and exit.

    **'-a'/'--append' (filename)** -- Open the specified file for
      append instead of using stdout.

    **'-b'/'--buffered-output'** -- Fully buffer processing output,
      including the file open itself.  This is helpful when, should an
      error occur, you wish that no output file be generated at all
      (for instance, when using EmPy in conjunction with make).  When
      specified, either the -o or -a options must be specified, and
      the -b option must precede them.  This can also be specified
      through the existence of the 'EMPY_BUFFERED_OUTPUT' environment
      variable.

    **'-f'/'--flatten'** -- Before processing, move the contents of
      the 'empy' pseudomodule into the globals, just as if
      'empy.flatten()' were executed immediately after starting the
      interpreter.  That is, *e.g.*, 'empy.include' can be referred to
      simply as 'include' when this flag is specified on the command
      line.  This can also be specified through the existence of the
      'EMPY_FLATTEN' environment variable.

    **'-i'/'--interactive'** -- After the main EmPy file has been
      processed, the state of the interpreter is left intact and
      further processing is done from stdin.  This is analogous to the
      Python interpreter's -i option, which allows interactive
      inspection of the state of the system after a main module is
      executed.  This behaves as expected when the main file is stdin
      itself.  This can also be specified through the existence of the
      'EMPY_INTERACTIVE' environment variable.

    **'-k'/'--suppress-errors'** -- Normally when an error is
      encountered, information about its location is printed and the
      EmPy interpreter exits.  With this option, when an error is
      encountered (except for keyboard interrupts), processing stops
      and the interpreter enters interactive mode, so the state of
      affairs can be assessed.  This is also helpful, for instance,
      when experimenting with EmPy in an interactive manner.  -k
      implies -i.

    **'-n'/'--no-override-stdout'** -- Do not override 'sys.stdout'
      with a proxy object which the EmPy system interacts with.  If
      suppressed, this means that side effect printing will not be
      captured and routed through the EmPy system.  However, if this
      option is specified, EmPy can support multithreading.

    **'-o'/'--output' (filename)** -- Open the specified file for
      output instead of using stdout.  If a file with that name
      already exists it is overwritten.

    **'-p'/'--prefix' (prefix)** -- Change the prefix used to detect
      expansions.  The argument is the one-character string that will
      be used as the prefix.  Note that whatever it is changed to, the
      way to represent the prefix literally is to double it, so if '$'
      is the prefix, a literal dollar sign is represented with '$$'.
      Note that if the prefix is changed to one of the secondary
      characters (those that immediately follow the prefix to indicate
      the type of action EmPy should take), it will not be possible to
      represent literal prefix characters by doubling them (*e.g.*, if
      the prefix were inadvisedly changed to '#' then '##' would
      already have to represent a comment, so '##' could not represent
      a literal '#').  This can also be specified through the
      'EMPY_PREFIX' environment variable.

    **'-r'/'--raw-errors'** -- Normally, EmPy catches Python
      exceptions and prints them alongside an error notation
      indicating the EmPy context in which it occurred.  This option
      causes EmPy to display the full Python traceback; this is
      sometimes helpful for debugging.  This can also be specified
      through the existence of the 'EMPY_RAW_ERRORS' environment
      variable.

    **'-u'/'--unicode'** -- Enable the Unicode subsystem.  This option
      only need be present if you wish to enable the Unicode subsystem
      with the defaults; any other Unicode-related option (starting
      with --unicode...) will also enable the Unicode subsystem.

    **'-D'/'--define' (assignment)** -- Execute a Python assignment of
      the form 'variable = expression'.  If only a variable name is
      provided (*i.e.*, the statement does not contain an '=' sign),
      then it is taken as being assigned to None.  The -D option is
      simply a specialized -E option that special cases the lack of an
      assignment operator.  Multiple -D options can be specified.

    **'-E'/'--execute' (statement)** -- Execute the Python (not EmPy)
      statement before processing any files.  Multiple -E options can
      be specified.

    **'-F'/'--execute-file' (filename)** -- Execute the Python (not
      EmPy) file before processing any files.  This is equivalent to
      '-E execfile("filename")' but provides a more readable context.
      Multiple -F options can be specified.

    **'-I'/'--import' (module)** -- Imports the specified module name
      before processing any files.  Multiple modules can be specified
      by separating them by commas, or by specifying multiple -I
      options.

    **'-P'/'--preprocess' (filename)** -- Process the EmPy file before
      processing the primary EmPy file on the command line.

    **'--binary'** -- Treat the file as a binary file, and read in
      chunks rather than line by line.  In this mode, the "line"
      indicator represents the number of bytes read, not the number of
      lines processed.

    **'--no-prefix'** -- Disable the prefixing system entirely; when
      specified, EmPy will not expand any markups.  This allows EmPy
      to merely act as a Unicode encoding translator..

    **'--pause-at-end'** -- If present, then 'raw_input' will be
      called at the end of processing.  Useful in systems where the
      output window would otherwise be closed by the operating
      system/window manager immediately after EmPy exited.

    **'--relative-path'** -- When present, the path the EmPy script
      being invoked is contained in will be prepended to 'sys.path'.
      This is analogous to Python's internal handling of 'sys.path'
      and scripts.  If input is from stdin ('-' for a filename or no
      filename is specified), then nothing is added to the path.

    **'--no-callback-error'** -- Do not consider it an error if the
      custom markup is invoked '@<...>' and there is no callback
      function registered for it.

    **'--chunk-size' (chunk)** -- Use the specific binary chunk size
      rather than the default; implies --binary.

    **'--unicode-encoding' (encoding)** -- Specify the Unicode
      encoding to be used for both input and output.

    **'--unicode-input-encoding' (encoding)** -- Specify the Unicode
      encoding to be used for input.

    **'--unicode-output-encoding' (encoding)** -- Specify the Unicode
      encoding to be used for output.

    **'--unicode-input-errors (errors)** -- Specify the Unicode error
      handling to be used for input.

    **'--unicode-errors (errors)** -- Specify the Unicode error
      handling to be used for both input and output.

    **'--unicode-output-errors (errors)** -- Specify the Unicode error
      handling to be used for output.


Environment variables

    EmPy also supports a few environment variables to predefine
    certain behaviors.  The settings chosen by environment variables
    can be overridden via command line arguments.  The following
    environment variables have meaning to EmPy:

    **'EMPY_OPTIONS'** -- If present, the contents of this environment
      variable will be treated as options, just as if they were
      entered on the command line, *before* the actual command line
      arguments are processed.  Note that these arguments are *not*
      processed by the shell, so quoting, filename globbing, and the
      like, will not work.

    **'EMPY_PREFIX'** -- If present, the value of this environment
      variable represents the prefix that will be used; this is
      equivalent to the -p command line option.

    **'EMPY_PSEUDO'** -- If present, the value of this environment
      variable represents the name of the pseudomodule that will be
      incorporated into every running EmPy system; this is equivalent
      to the -m command line option.

    **'EMPY_FLATTEN'** -- If defined, this is equivalent to including
      -f on the command line.

    **'EMPY_RAW_ERRORS'** -- If defined, this is equivalent to
      including -r on the command line.

    **'EMPY_INTERACTIVE'** -- If defined, this is equivalent to
      including -i on the command line.

    **'EMPY_BUFFERED_OUTPUT'** -- If defined, this is equivalent to
      including -b on the command line.

    **'EMPY_UNICODE'** -- If defined, this is equivalent to including
      -u on the command line.

    **'EMPY_UNICODE_INPUT_ENCODING'** -- If present, the value of this
      environment variable indicates the name of the Unicode input
      encoding to be used.  This is equivalent to the
      --unicode-input-encoding command line option.

    **'EMPY_UNICODE_OUTPUT_ENCODING'** -- If present, the value of
      this environment variable indicates the name of the Unicode
      output encoding to be used.  This is equivalent to the
      --unicode-output-encoding command line option.

    **'EMPY_UNICODE_INPUT_ERRORS'** -- If present, the value of this
      environment variable indicates the name of the error handler to
      be used for input.  This is equivalent to the
      --unicode-input-errors command line option.

    **'EMPY_UNICODE_OUTPUT_ERRORS'** -- If present, the value of this
      environment variable indicates the name of the error handler to
      be used for output.  This is equivalent to the
      --unicode-output-errors command line option.


Examples and testing EmPy

    See the sample EmPy file 'sample.em' which is included with the
    distribution.  Run EmPy on it by typing something like::

         ./em.py sample.em

    and compare the results and the sample source file side by side.
    The sample content is intended to be self-documenting, and even an
    introduction to the basic features of EmPy while simultaneously
    exercising them.

    The file 'sample.bench' is the benchmark output of the sample.
    Running the EmPy interpreter on the provided 'sample.em' file
    should produce precisely the same results.  You can run the
    provided test script to see if your EmPy environment is behaving
    as expected (presuming a Unix-like operating system)::

        ./test.sh

    By default this will test with the first Python interpreter
    available in the path; if you want to test with another
    interpreter, you can provide it as the first argument on the
    command line, *e.g.*::

        ./test.sh python2.1
        ./test.sh /usr/bin/python1.5
        ./test.sh jython

    A more comprehensive test suite and set of real-world examples is
    planned for a future version.


Embedding EmPy

    For atomic applications, the 'expand' function is provided (the
    extra keyword arguments passed in are treated as locals)::

        import em
        print em.expand("@x + @y is @(x + y).", x=2, y=3)

    One can specify a globals dictionary and all the other interpreter
    options (below) as well.  One can specify a globals dictionary
    that will be used if one wants persistence::

        import em
        g = {}
        em.expand("@{x = 10}", g)
        print em.expand("x is @x.", g)

    The standalone 'expand' function, however, creates and destroys an
    'Interpreter' instance each time it is called.  For repeated
    expansions, this can be expensive.  Instead, you will probably
    want to use the full-fledged features of embedding.  An EmPy
    interpreter can be created with as code as simple as::

        import em
        interpreter = em.Interpreter()
        # The following prints the results to stdout:
        interpreter.string("@{x = 123}@x\n")
        # This expands to the same thing, but puts the results as a
        # string in the variable result:
        result = interpreter.expand("@{x = 123}@x\n")
        # This just prints the value of x directly:
        print interpreter.globals['x']
        # Process an actual file (and output to stdout):
        interpreter.file(open('/path/to/some/file'))
        interpreter.shutdown() # this is important; see below

    One can capture the output of a run in something other than stdout
    by specifying the *output* parameter::

        import em, StringIO
        output = StringIO.StringIO()
        interpreter = em.Interpreter(output=output)
        # Do something.
        interpreter.file(open('/path/to/some/file'))
        interpreter.shutdown() # again, this is important; see below
        print output.getvalue() # this is the result from the session

    When you are finished with your interpreter, it is important to
    call its shutdown method::

        interpreter.shutdown()

    This will ensure that the interpreter cleans up all its overhead,
    entries in the 'sys.stdout' proxy, and so forth.  It is usually
    advisable that this be used in a try...finally clause::

        interpreter = em.Interpreter(...)
        try:
            ...
        finally:
            interpreter.shutdown()

    The 'em.Interpreter' constructor takes the following arguments;
    all are optional.  Since options may be added in the future, it is
    highly recommended that the constructor be invoked via keyword
    arguments, rather than assuming their order.  The arguments are:

    *output* -- The output file which the interpreter will be sending
     all its processed data to.  This need only be a file-like object;
     it need not be an actual file.  If omitted, 'sys.__stdout__' is
     used.

    *argv* -- An argument list analogous to 'sys.argv', consisting of
     the script name and zero or more arguments.  These are available
     to executing interpreters via 'empy.argv' and 'empy.args'.  If
     omitted, a non-descript script name is used with no arguments.

    *prefix* -- The prefix (a single-character string).  Defaults to
     '@'.  It is an error for this to be anything other than one
     character.

    *pseudo* -- The name (string) of the pseudmodule.  Defaults to
     'empy'.

    *options* -- A dictionary of options that can override the default
     behavior of the interpreter.  The names of the options are
     constant names ending in '_OPT' and their defaults are given in
     'Interpreter.DEFAULT_OPTIONS'.

    *globals* -- By default, interpreters begin with a pristine
     dictionary of globals (except, of course, for the 'empy'
     pseudomodule).  Specifying this argument will allow the globals
     to start with more.

    *hooks* -- A sequence of hooks (or 'None' for none) to register
     with the interpreter at startup.  Hooks can, of course, be added
     after the fact, but this allows the hooks to intercept the
     'atStartup' event (otherwise, the startup event would already
     have occurred by the time new hooks could be registered)..

    Many things can be done with EmPy interpreters; for the full
    developer documentation, see the generated documentation for the
    'em' module.


Interpreter options

    The following options (passed in as part of the options dictionary
    to the Interpreter constructor) have the following meanings.  The
    defaults are shown below and are also indicated in an
    'Interpreter.DEFAULT_OPTIONS' dictionary.

    **'BANGPATH_OPT'** -- Should a bangpath ('#!') as the first line
      of an EmPy file be treated as if it were an EmPy comment?  Note
      that '#!' sequences starting lines or appearing anywhere else in
      the file are untouched regardless of the value of this option.
      Default: true.

    **'BUFFERED_OPT'** -- Should an 'abort' method be called upon
      failure?  This relates to the fully-buffered option, where all
      output can be buffered including the file open; this option only
      relates to the interpreter's behavior *after* that proxy file
      object has been created.  Default: false.

    **'RAW_OPT'** -- Should errors be displayed as raw Python errors
      (that is, the exception is allowed to propagate through to the
      toplevel so that the user gets a standard Python traceback)?
      Default: false.

    **'EXIT_OPT'** -- Upon an error, should execution continue
      (although the interpreter stacks will be purged)?  Note that
      even in the event this is set, the interpreter will halt upon
      receiving a 'KeyboardInterrupt'.  Default: true.

    **'FLATTEN_OPT'** -- Upon initial startup, should the 'empy'
      pseudomodule namespace be flattened, *i.e.*, should
      'empy.flatten' be called?  Note this option only has an effect
      when the interpreter is first created; thereafter it is
      ignored.  Default: false.

    **'OVERRIDE_OPT'** -- Should the 'sys.stdout' object be overridden
      with a proxy object?  If not, side effect output cannot be
      captured by the EmPy system, but EmPy will support
      multithreading.  Default: true.

    **'CALLBACK_OPT'** -- If a callback is invoked when none has yet
      been registered, should an error be raised or should the
      situation be ignored?  Default: true.


Data flow

    **input -> interpreter -> diversions -> filters -> output**

    Here, in summary, is how data flows through a working EmPy system:

    1. Input comes from a source, such an .em file on the command
       line, or via an 'empy.include' statement.

    2. The interpreter processes this material as it comes in,
       expanding EmPy expansions as it goes.

    3. After interpretation, data is then sent through the diversion
       layer, which may allow it directly through (if no diversion is
       in progress) or defer it temporarily.  Diversions that are
       recalled initiate from this point.

    4. Any filters in place are then used to filter the data and
       produce filtered data as output.

    5. Finally, any material surviving this far is sent to the output
       stream.  That stream is stdout by default, but can be changed
       with the -o or -a options, or may be fully buffered with the -b
       option (that is, the output file would not even be opened until
       the entire system is finished).


Author's notes

    I originally conceived EmPy as a replacement for my "Web
    templating system", http://www.alcyone.com/max/info/m4.html which
    uses "m4", http://www.seindal.dk/rene/gnu/ (a general
    macroprocessing system for Unix).

    Most of my Web sites include a variety of m4 files, some of which
    are dynamically generated from databases, which are then scanned
    by a cataloging tool to organize them hierarchically (so that,
    say, a particular m4 file can understand where it is in the
    hierarchy, or what the titles of files related to it are without
    duplicating information); the results of the catalog are then
    written in database form as an m4 file (which every other m4 file
    implicitly includes), and then GNU make converts each m4 to an
    HTML file by processing it.

    As the Web sites got more complicated, the use of m4 (which I had
    originally enjoyed for the challenge and abstractness) really
    started to become an impediment to serious work; while I am very
    knowledgeable about m4 -- having used it for for so many years --
    getting even simple things done with it is awkward and difficult.
    Worse yet, as I started to use Python more and more over the
    years, the cataloging programs which scanned the m4 and built m4
    databases were migrated to Python and made almost trivial, but
    writing out huge awkward tables of m4 definitions simply to make
    them accessible in other m4 scripts started to become almost
    farcical -- especially when coupled with the difficulty in getting
    simple things done in m4.

    It occurred to me what I really wanted was an all-Python solution.
    But replacing what used to be the m4 files with standalone Python
    programs would result in somewhat awkward programs normally
    consisting mostly of unprocessed text punctuated by small portions
    where variables and small amounts of code need to be substituted.
    Thus the idea was a sort of inverse of a Python interpreter: a
    program that normally would just pass text through unmolested, but
    when it found a special signifier would execute Python code in a
    normal environment.  I looked at existing Python templating
    systems, and didn't find anything that appealed to me -- I wanted
    something where the desired markups were simple and unobtrusive.
    After considering between choices of signifiers, I settled on '@'
    and EmPy was born.

    As I developed the tool, I realized it could have general appeal,
    even to those with widely varying problems to solve, provided the
    core tool they needed was an interpreter that could embed Python
    code inside templated text.  As I continue to use the tool, I have
    been adding features as unintrusively as possible as I see areas
    that can be improved.

    A design goal of EmPy is that its feature set should work on
    several levels; at each level, if the user does not wish or need
    to use features from another level, they are under no obligation
    to do so.  If you have no need of diversions, for instance, you
    are under no obligation to use them.  If significators will not
    help you organize a set of EmPy scripts globally, then you need
    not use them.  New features that are being added are whenever
    possible transparently backward compatible; if you do not need
    them, their introduction should not affect you in any way.  The
    use of unknown prefix sequences results in errors, guaranteeing
    that they are reserved for future use.


Glossary

    **control** -- A control markup, used to direct high-level control
      flow within an EmPy session.  Control markups are expressed with
      the '@[...]' notation.

    **diversion** -- A process by which output is deferred, and can be
      recalled later on demand, multiple times if necessary.

    **document** -- The abstraction of an EmPy document as used by a
      processor.

    **escape** -- A markup designed to expand to a single (usually
      non-printable) character, similar to escape sequences in C or
      other languages.

    **expansion** -- The process of processing EmPy markups and
      producing output.

    **expression** -- An expression markup represents a Python
      expression to be evaluated, and replaced with the 'str' of its
      value.  Expression markups are expressed with the '@(...)'
      notation.

    **filter** -- A file-like object which can be chained to other
      objects (primarily the final stream) and can buffer, alter, or
      manipulate in any way the data sent.  Filters can also be
      chained together in arbitrary order.

    **globals** -- The dictionary (or dictionary-like object) which
      resides inside the interpreter and holds the currently-defined
      variables.

    **hook** -- A callable object that can be registered in a
      dictionary, and which will be invoked before, during, or after
      certain internal operations, identified by name with a string.

    **interpreter** -- The application (or class instance) which
      processes EmPy markup.

    **markup** -- EmPy substitutions set off with a prefix and
      appropriate delimeters.

    **output** -- The final destination of the result of processing an
      EmPy file.

    **prefix** -- The ASCII character used to set off an expansions.
      By default, '@'.

    **processor** -- An extensible system which processes a group of
      EmPy files, usually arranged in a filesystem, and scans them for
      significators.

    **pseudomodule** -- The module-like object named 'empy' which is
      exposed internally inside every EmPy system.

    **shortcut** -- A special object which takes the place of an
      instance of the 'Filter' class, to represent a special form of
      filter.  These include 0 for a null filter, a callable (function
      or lambda) to represent a callable filter, or a 256-character
      string which represents a translation filter.

    **significator** -- A special form of an assignment markup in EmPy
      which can be easily parsed externally, primarily designed for
      representing uniform assignment across a collection of files.
      Significators are indicated with the '@%' markup.

    **statement** -- A line of code that needs to be executed;
      statements do not have return values.  In EmPy, statements are
      set off with '@{...}'.


Acknowledgements

    Questions, suggestions, bug reports, evangelism, and even
    complaints from many people have helped make EmPy what it is
    today.  Some, but by no means all, of these people are (in
    alphabetical order by surname):

    - Biswapesh Chattopadhyay

    - Beni Cherniavsky

    - Dr. S. Candelaria de Ram

    - Eric Eide

    - Dinu Gherman

    - Grzegorz Adam Hankiewicz

    - Bohdan Kushnir

    - Robert Kroeger

    - Kouichi Takahashi

    - Ville Vainio


Known issues and caveats

    - EmPy was primarily intended for static processing of documents,
      rather than dynamic use, and hence speed of processing was not
      the primary consideration in its design.

    - EmPy is not threadsafe by default.  This is because of the need
      for EmPy to override the 'sys.stdout' file with a proxy object
      which can capture effects of 'print' and other spooling to
      stdout.  This proxy can be suppressed with the -n option, which
      will result in EmPy being unable to do anything meaningful with
      this output, but will allow EmPy to be threadsafe.

    - To function properly, EmPy must override 'sys.stdout' with a
      proxy file object, so that it can capture output of side effects
      and support diversions for each interpreter instance.  It is
      important that code executed in an environment *not* rebind
      'sys.stdout', although it is perfectly legal to invoke it
      explicitly (*e.g.*, '@sys.stdout.write("Hello world\n")').  If
      one really needs to access the "true" stdout, then use
      'sys.__stdout__' instead (which should also not be rebound).
      EmPy uses the standard Python error handlers when exceptions are
      raised in EmPy code, which print to 'sys.stderr'.

    - Due to Python's curious handling of the 'print' statement --
      particularly the form with a trailing comma to suppress the
      final newline -- mixing statement expansions using prints inline
      with unexpanded text will often result in surprising behavior,
      such as extraneous (sometimes even deferred!) spaces.  This is a
      Python "feature," and occurs in non-EmPy applications as well;
      for finer control over output formatting, use 'sys.stdout.write'
      or 'empy.interpreter.write' directly.

    - The 'empy' "module" exposed through the EmPy interface (*e.g.*,
      '@empy') is an artificial module.  It cannot be imported with
      the 'import' statement (and shouldn't -- it is an artifact of
      the EmPy processing system and does not correspond to any
      accessible .py file).

    - For an EmPy statement expansion all alone on a line, *e.g.*,
      '@{a = 1}', note that this will expand to a blank line due to
      the newline following the closing curly brace.  To suppress this
      blank line, use the symmetric convention '@{a = 1}@'.

    - When using EmPy with make, note that partial output may be
      created before an error occurs; this is a standard caveat when
      using make.  To avoid this, write to a temporary file and move
      when complete, delete the file in case of an error, use the -b
      option to fully buffer output (including the open), or (with GNU
      make) define a '.DELETE_ON_ERROR' target.

    - 'empy.identify' tracks the context of executed *EmPy* code, not
      Python code.  This means that blocks of code delimited with '@{'
      and '}' will identify themselves as appearing on the line at
      which the '}' appears, and that pure Python code executed via
      the -D, -E and -F command line arguments will show up as all taking
      place on line 1.  If you're tracking errors and want more
      information about the location of the errors from the Python
      code, use the -r command line option, which will provide you
      with the full Python traceback.

    - The conditional form of expression expansion '@(...?...!...)'
      allows the use of a colon instead of an exclamation point,
      *e.g.*, '@(...?...:...)'.  This behavior is supported for
      backward compatibility, but is deprecated.  Due to an oversight,
      the colon was a poor choice since colons can appear legally in
      expressions (*e.g.*, dictionary literals or lambda expressions).

    - The '@[try]' construct only works with Python exceptions derived
      from 'Exception'.  It is not able to catch string exceptions.

    - The '@[for]' variable specification supports tuples for tuple
      unpacking, even recursive tuples.  However, it is limited in
      that the names included may only be valid Python identifiers,
      not arbitrary Python lvalues.  Since the internal Python
      mechanism is very rarely used for this purpose (*e.g.*, 'for (x,
      l[0], q.a) in sequence'), it is not thought to be a significant
      limitation.


Wish list

    Here are some random ideas for future revisions of EmPy.  If any
    of these are of particular interest to you, your input would be
    appreciated.

    - Some real-world examples should really be included for
      demonstrating the power and expressiveness of EmPy first-hand.

    - More extensive help (rather than a ridiculously long README),
      probably inherently using the EmPy system itself for building to
      HTML and other formats, thereby acting as a help facility and a
      demonstration of the working system.

    - A "trivial" mode, where all the EmPy system does is scan for
      simple symbols to replace them with evaluations/executions,
      rather than having to do the contextual scanning it does now.
      This has the down side of being much less configurable and
      powerful but the upside of being extremely efficient.

    - A "debug" mode, where EmPy prints the contents of everything
      it's about to evaluate (probably to stderr) before it does?

    - The ability to funnel all code through a configurable 'RExec'
      for user-controlled security control.  This would probably
      involve abstracting the execution functionality outside of the
      interpreter.  [This suggestion is on hold until the
      rexec/Bastion exploits are worked out.]

    - Optimized handling of processing would be nice for the
      possibility of an Apache module devoted to EmPy processing.

    - An EmPy emacs mode.

    - An optimization of offloading diversions to files when they
      become truly huge.  (This is made possible by the abstraction of
      the 'Diversion' class.)

    - Support for mapping filters (specified by dictionaries).

    - Support for some sort of batch processing, where several EmPy
      files can be listed at once and all of them evaluated with the
      same initial (presumably expensive) environment.
      'empy.saveGlobals' and 'empy.restoreGlobals' have been
      introduced as a partial solution, but they need to be made more
      robust.

    - A more elaborate interactive mode, perhaps with a prompt and
      readline support.

    - A StructuredText and/or reStructuredText filter would be quite
      useful, as would SGML/HTML/XML/XHTML, s-expression, Python,
      etc. auto-indenter filters.

    - An indexing filter, which can process text and pick out
      predefined keywords and thereby setup links to them.

    - The ability to rerun diverted material back through the
      interpreter.  (This can be done, awkwardly, by manually creating
      a filter which itself contains an interpreter, but it might be
      helpful if this was an all-in-one operation.)

    - A caching system that stores off the compilations of repeated
      evaluations and executions so that in a persistent environment
      the same code does not have to be repeatedly evaluated/executed.
      This would probably be a necessity in an Apache module-based
      solution.  Perhaps caching even to the point of generating pure
      PyWM bytecode?

    - An option to change the format of the standard EmPy errors in a
      traceback.

    - Support for some manner of implicitly processed /etc/empyrc
      and/or ~/.empyrc file, and of course an option to inhibit its
      processing.  This can already be accomplished (and with greater
      control) via use of EMPY_OPTIONS, though.

    - More uniform handling of the preprocessing directives (-I, -D,
      -E, -F, and -P), probably mapping directly to methods in the
      'Interpreter' class.

    - Support for integration with mod_python.

    - In simple expressions, a '{...}' suffix has no meaning in Python
      (*e.g.*, in Python, '@x(...)' is a call, '@x[...]' is
      subscription, but '@x{...}' is illegal).  This could be
      exploited by having a '{...}' suffix in a simple expression
      representing an encapsulation of an expanded string; *e.g.*,
      '@bullet{There are @count people here}' would be equivalent to
      '@bullet(empy.expand("There are @count people here",
      locals()))}'.

    - A tool to collect significator information from a hierarchy of
      .em files and put them in a database form available for
      individual scripts would be extremely useful -- this tool should
      be extensible so that users can use it to, say, build ordered
      hierarchies of their EmPy files by detecting contextual
      information like application-specific links to other EmPy
      documents.

    - Extensions of the basic EmPy concepts to projects for other
      interpreted languages, such as Java, Lua, Ruby, and/or Perl.

    - Ignore 'SystemExit' when doing error handling, letting the
      exception progagate up?  So far no one seems to worry about
      this; deliberately exiting early in a template seems to be an
      unlikely occurrence.  (Furthermore, there are the 'os.abort' and
      'os._exit' facilities for terminating without exception
      propagation.)

    - A new markup which is the equivalent of '$...:...$' in source
      control systems, where the left-hand portion represents a
      keyword and the right-hand portion represents its value which is
      substituted in by the EmPy system.

    - The ability to obtain the filename (if relevant) and mode of the
      primary output file.

    - The ability to redirect multiple streams of output; not
      diversions, but rather the ability to write to one file and then
      another.  Since output would be under the EmPy script's control,
      this would imply a useful --no-output option, where by default
      no output is written.  This would also suggest the usefulness of
      all the output file delegates (diversions, filters, abstract
      files, etc.) passing unrecognized method calls all the way down
      to underlying file object.

    - In addition to the em.py script, an additional support library
      (non-executable) should be included which includes ancillary
      functionality for more advanced features, but which is not
      necessary to use EmPy in its basic form as a standalone
      executable.  Such features would include things like
      significator processing, metadata scanning, and advanced
      prompting systems.


Release history

    - 3.3.3; 2017 Feb 12.  Fix for 'defined' call.

    - 3.3.2; 2014 Jan 24.  Additional fix for source compatibility
      between 2.x and 3.0.

    - 3.3.1; 2014 Jan 22.  Source compatibility for 2.x and 3.0;
      1.x and Jython compatibility dropped.

    - 3.3; 2003 Oct 27.  Custom markup '@<...>'; remove separate
      pseudomodule instance for greater transparency; deprecate
      'interpreter' attribute of pseudomodule; deprecate auxiliary
      class name attributes associated with pseudomodule in
      preparation for separate support library in 4.0; add
      --no-callback-error and --no-bangpath-processing command line
      options; add 'atToken' hook.

    - 3.2; 2003 Oct 7.  Reengineer hooks support to use hook
      instances; add -v option; add --relative-path option; reversed
      PEP 317 style; modify Unicode support to give less confusing
      errors in the case of unknown encodings and error handlers;
      relicensed under LGPL.

    - 3.1.1; 2003 Sep 20.  Add literal '@"..."' markup; add
      --pause-at-end command line option; fix improper globals
      collision error via the 'sys.stdout' proxy.

    - 3.1; 2003 Aug 8.  Unicode support (Python 2.0 and above); add
      Document and Processor helper classes for processing
      significators; add --no-prefix option for suppressing all
      markups.

    - 3.0.4; 2003 Aug 7.  Implement somewhat more robust lvalue
      parsing for '@[for]' construct (thanks to Beni Cherniavsky for
      inspiration).

    - 3.0.3; 2003 Jul 9.  Fix bug regarding recursive tuple unpacking
      using '@[for]'; add 'empy.saveGlobals', 'empy.restoreGlobals',
      and 'empy.defined' functions.

    - 3.0.2; 2003 Jun 19.  '@?' and '@!' markups for changing the
      current context name and line, respectively; add 'update' method
      to interpreter; new and renamed context operations,
      'empy.setContextName', 'empy.setContextLine',
      'empy.pushContext', 'empy.popContext'.

    - 3.0.1; 2003 Jun 9.  Fix simple bug preventing command line
      preprocessing directives (-I, -D, -E, -F, -P) from executing
      properly; defensive PEP 317 compliance [defunct].

    - 3.0; 2003 Jun 1.  Control markups with '@[...]'; remove
      substitutions (use control markups instead); support
      '@(...?...!...)' for conditional expressions in addition to the
      now-deprecated '@(...?...:...)' variety; add acknowledgements
      and glossary sections to documentation; rename buffering option
      back to -b; add -m option and 'EMPY_PSEUDO' environment variable
      for changing the pseudomodule name; add -n option and
      'EMPY_NO_OVERRIDE' environment variable for suppressing
      'sys.stdout' proxy; rename main error class to 'Error'; add
      standalone 'expand' function; add --binary and --chunk-size
      options; reengineer parsing system to use Tokens for easy
      extensibility; safeguard curly braces in simple expressions
      (meaningless in Python and thus likely a typographical error) by
      making them a parse error; fix bug involving custom Interpreter
      instances ignoring globals argument; distutils support.

    - 2.3; 2003 Feb 20.  Proper and full support for concurrent and
      recursive interpreters; protection from closing the true stdout
      file object; detect edge cases of interpreter globals or
      'sys.stdout' proxy collisions; add globals manipulation
      functions 'empy.getGlobals', 'empy.setGlobals', and
      'empy.updateGlobals' which properly preserve the 'empy'
      pseudomodule; separate usage info out into easily accessible
      lists for easier presentation; have -h option show simple usage
      and -H show extened usage; add 'NullFile' utility class.

    - 2.2.6; 2003 Jan 30.  Fix a bug in the 'Filter.detach' method
      (which would not normally be called anyway).

    - 2.2.5; 2003 Jan 9.  Strip carriage returns out of executed code
      blocks for DOS/Windows compatibility.

    - 2.2.4; 2002 Dec 23.  Abstract Filter interface to use methods
      only; add '@[noop: ...]' substitution for completeness and block
      commenting [defunct].

    - 2.2.3; 2002 Dec 16.  Support compatibility with Jython by
      working around a minor difference between CPython and Jython in
      string splitting.

    - 2.2.2; 2002 Dec 14.  Include better docstrings for pseudomodule
      functions; segue to a dictionary-based options system for
      interpreters; add 'empy.clearAllHooks' and 'empy.clearGlobals';
      include a short documentation section on embedding interpreters;
      fix a bug in significator regular expression.

    - 2.2.1; 2002 Nov 30.  Tweak test script to avoid writing
      unnecessary temporary file; add 'Interpreter.single' method;
      expose 'evaluate', 'execute', 'substitute' [defunct], and
      'single' methods to the pseudomodule; add (rather obvious)
      'EMPY_OPTIONS' environment variable support; add
      'empy.enableHooks' and 'empy.disableHooks'; include optimization
      to transparently disable hooks until they are actually used.

    - 2.2; 2002 Nov 21.  Switched to -V option for version
      information; 'empy.createDiversion' for creating initially empty
      diversion; direct access to diversion objects with
      'empy.retrieveDiversion'; environment variable support; removed
      --raw long argument (use --raw-errors instead); added quaternary
      escape code (well, why not).

    - 2.1; 2002 Oct 18.  'empy.atExit' registry separate from hooks to
      allow for normal interpreter support; include a benchmark sample
      and test.sh verification script; expose 'empy.string' directly;
      -D option for explicit defines on command line; remove
      ill-conceived support for '@else:' separator in '@[if ...]'
      substitution [defunct] ; handle nested substitutions properly
      [defunct] ; '@[macro ...]' substitution for creating recallable
      expansions [defunct].

    - 2.0.1; 2002 Oct 8.  Fix missing usage information; fix
      after_evaluate hook not getting called; add 'empy.atExit' call
      to register values.

    - 2.0; 2002 Sep 30.  Parsing system completely revamped and
      simplified, eliminating a whole class of context-related bugs;
      builtin support for buffered filters; support for registering
      hooks; support for command line arguments; interactive mode with
      -i; significator value extended to be any valid Python
      expression.

    - 1.5.1; 2002 Sep 24.  Allow '@]' to represent unbalanced close
      brackets in '@[...]' markups [defunct].

    - 1.5; 2002 Sep 18.  Escape codes ('@\...'); conditional and
      repeated expansion substitutions [defunct] ; replaced with control
      markups]; fix a few bugs involving files which do not end in
      newlines.

    - 1.4; 2002 Sep 7.  Fix bug with triple quotes; collapse
      conditional and protected expression syntaxes into the single
      generalized '@(...)' notation; 'empy.setName' and 'empy.setLine'
      functions [deprecated] ; true support for multiple concurrent
      interpreters with improved sys.stdout proxy; proper support for
      'empy.expand' to return a string evaluated in a subinterpreter
      as intended; merged Context and Parser classes together, and
      separated out Scanner functionality.

    - 1.3; 2002 Aug 24.  Pseudomodule as true instance; move toward
      more verbose (and clear) pseudomodule functions; fleshed out
      diversion model; filters; conditional expressions; protected
      expressions; preprocessing with -P (in preparation for
      possible support for command line arguments).

    - 1.2; 2002 Aug 16.  Treat bangpaths as comments; 'empy.quote' for
      the opposite process of 'empy.expand'; significators ('@%...'
      sequences); -I option; -f option; much improved documentation.

    - 1.1.5; 2002 Aug 15.  Add a separate 'invoke' function that can be
      called multiple times with arguments to simulate multiple runs.

    - 1.1.4; 2002 Aug 12.  Handle strings thrown as exceptions
      properly; use getopt to process command line arguments; cleanup
      file buffering with AbstractFile; very slight documentation and
      code cleanup.

    - 1.1.3; 2002 Aug 9.  Support for changing the prefix from within
      the 'empy' pseudomodule.

    - 1.1.2; 2002 Aug 5.  Renamed buffering option [defunct], added -F
      option for interpreting Python files from the command line,
      fixed improper handling of exceptions from command line options
      (-E, -F).

    - 1.1.1; 2002 Aug 4.  Typo bugfixes; documentation clarification.

    - 1.1; 2002 Aug 4.  Added option for fully buffering output
      (including file opens), executing commands through the command
      line; some documentation errors fixed.

    - 1.0; 2002 Jul 23.  Renamed project to EmPy.  Documentation and
      sample tweaks; added 'empy.flatten'.  Added -a option.

    - 0.3; 2002 Apr 14.  Extended "simple expression" syntax,
      interpreter abstraction, proper context handling, better error
      handling, explicit file inclusion, extended samples.

    - 0.2; 2002 Apr 13.  Bugfixes, support non-expansion of Nones,
      allow choice of alternate prefix.

    - 0.1.1; 2002 Apr 12.  Bugfixes, support for Python 1.5.x, add -r
      option.

    - 0.1; 2002 Apr 12.  Initial early access release.


Author

    This module was written by "Erik Max Francis",
    http://www.alcyone.com/max/.  If you use this software, have
    suggestions for future releases, or bug reports, "I'd love to hear
    about it", mailto:software@alcyone.com.

    Even if you try out EmPy for a project and find it unsuitable, I'd
    like to know what stumbling blocks you ran into so they can
    potentially be addressed in a future version.


Version

    Version 3.3.3 $Date: 2014-01-24 13:39:38 -0800 (Fri, 24 Jan 2014) $ $Author: max $