src/tools/python.jam

# Copyright 2004 Vladimir Prus.
# Distributed under the Boost Software License, Version 1.0. (See
# accompanying file LICENSE_1_0.txt or copy at
# http://www.boost.org/LICENSE_1_0.txt)

# Support for Python and the the Boost.Python library.
#
# This module defines
#
# - a project 'python' with a target 'python' in it, that corresponds to the
#   python library
#
# - a main target rule 'python-extension' which can be used to build a python
#   extension.
#
# Extensions that use Boost.Python must explicitly link to it.

import type ;
import testing ;
import generators ;
import project ;
import errors ;
import targets ;
import "class" : new ;
import os ;
import common ;
import toolset ;
import regex ;
import numbers ;
import string ;
import property ;
import sequence ;
import path ;
import feature ;
import set ;
import builtin ;


# Make this module a project.
project.initialize $(__name__) ;
project python ;

# Save the project so that if 'init' is called several times we define new
# targets in the python project, not in whatever project we were called by.
.project = [ project.current ] ;

# Dynamic linker lib. Necessary to specify it explicitly on some platforms.
lib dl ;
# This contains 'openpty' function need by python. Again, on some system need to
# pass this to linker explicitly.
lib util ;
# Python uses pthread symbols.
lib pthread ;
# Extra library needed by phtread on some platforms.
lib rt ;

# The pythonpath feature specifies additional elements for the PYTHONPATH
# environment variable, set by run-pyd. For example, pythonpath can be used to
# access Python modules that are part of the product being built, but are not
# installed in the development system's default paths.
feature.feature pythonpath : : free optional path ;

# Initializes the Python toolset. Note that all parameters are optional.
#
# - version -- the version of Python to use. Should be in Major.Minor format,
#   for example 2.3.  Do not include the subminor version.
#
# - cmd-or-prefix: Preferably, a command that invokes a Python interpreter.
#   Alternatively, the installation prefix for Python libraries and includes. If
#   empty, will be guessed from the version, the platform's installation
#   patterns, and the python executables that can be found in PATH.
#
# - includes: the include path to Python headers. If empty, will be guessed.
#
# - libraries: the path to Python library binaries. If empty, will be guessed.
#   On MacOS/Darwin, you can also pass the path of the Python framework.
#
# - condition: if specified, should be a set of properties that are matched
#   against the build configuration when Boost.Build selects a Python
#   configuration to use.
#
# - extension-suffix: A string to append to the name of extension modules before
#   the true filename extension.  Ordinarily we would just compute this based on
#   the value of the <python-debugging> feature. However ubuntu's python-dbg
#   package uses the windows convention of appending _d to debug-build extension
#   modules. We have no way of detecting ubuntu, or of probing python for the
#   "_d" requirement, and if you configure and build python using
#   --with-pydebug, you'll be using the standard *nix convention. Defaults to ""
#   (or "_d" when targeting windows and <python-debugging> is set).
#
# Example usage:
#
#   using python : 2.3 ;
#   using python : 2.3 : /usr/local/bin/python ;
#
rule init ( version ? : cmd-or-prefix ? : includes * : libraries ?
    : condition * : extension-suffix ? )
{
    project.push-current $(.project) ;

    debug-message Configuring python... ;
    for local v in version cmd-or-prefix includes libraries condition
    {
        if $($(v))
        {
            debug-message "  user-specified "$(v): \"$($(v))\" ;
        }
    }

    configure $(version) : $(cmd-or-prefix) : $(includes) : $(libraries) : $(condition) : $(extension-suffix) ;

    project.pop-current ;
}

# A simpler version of SHELL that grabs stderr as well as stdout, but returns
# nothing if there was an error.
#
local rule shell-cmd ( cmd )
{
    debug-message running command '$(cmd)" 2>&1"' ;
    x = [ SHELL $(cmd)" 2>&1" : exit-status ] ;
    if $(x[2]) = 0
    {
        return $(x[1]) ;
    }
    else
    {
        return ;
    }
}


# Try to identify Cygwin symlinks. Invoking such a file directly as an NT
# executable from a native Windows build of bjam would be fatal to the bjam
# process. One /can/ invoke them through sh.exe or bash.exe, if you can prove
# that those are not also symlinks. ;-)
#
# If a symlink is found returns non-empty; we try to extract the target of the
# symlink from the file and return that.
#
# Note: 1. only works on NT  2. path is a native path.
local rule is-cygwin-symlink ( path )
{
    local is-symlink = ;

    # Look for a file with the given path having the S attribute set, as cygwin
    # symlinks do. /-C means "do not use thousands separators in file sizes."
    local dir-listing = [ shell-cmd "DIR /-C /A:S \""$(path)"\"" ] ;

    if $(dir-listing)
    {
        # Escape any special regex characters in the base part of the path.
        local base-pat = [ regex.escape $(path:D=) : ].[()*+?|\\$^ : \\ ] ;

        # Extract the file's size from the directory listing.
        local size-of-system-file = [ MATCH "([0-9]+) "$(base-pat) : $(dir-listing) : 1 ] ;

        # If the file has a reasonably small size, look for the special symlink
        # identification text.
        if $(size-of-system-file) && [ numbers.less $(size-of-system-file) 1000 ]
        {
            local link = [ SHELL "FIND /OFF \"!<symlink>\" \""$(path)"\" 2>&1" ] ;
            if $(link[2]) != 0
            {
                local nl = "

" ;
                is-symlink = [ MATCH ".*!<symlink>([^"$(nl)"]*)" : $(link[1]) : 1 ] ;
                if $(is-symlink)
                {
                    is-symlink = [ *nix-path-to-native $(is-symlink) ] ;
                    is-symlink = $(is-symlink:R=$(path:D)) ;
                }

            }
        }
    }
    return $(is-symlink) ;
}


# Append ext to each member of names that does not contain '.'.
#
local rule default-extension ( names * : ext * )
{
    local result ;
    for local n in $(names)
    {
        switch $(n)
        {
            case *.* : result += $(n) ;
            case * : result += $(n)$(ext) ;
        }
    }
    return $(result) ;
}


# Tries to determine whether invoking "cmd" would actually attempt to launch a
# cygwin symlink.
#
# Note: only works on NT.
#
local rule invokes-cygwin-symlink ( cmd )
{
    local dirs = $(cmd:D) ;
    if ! $(dirs)
    {
        dirs = . [ os.executable-path ] ;
    }
    local base = [ default-extension $(cmd:D=) : .exe .cmd .bat ] ;
    local paths = [ GLOB $(dirs) : $(base) ] ;
    if $(paths)
    {
        # Make sure we have not run into a Cygwin symlink. Invoking such a file
        # as an NT executable would be fatal for the bjam process.
        return [ is-cygwin-symlink $(paths[1]) ] ;
    }
}


local rule debug-message ( message * )
{
    if --debug-configuration in [ modules.peek : ARGV ]
    {
        ECHO notice: [python-cfg] $(message) ;
    }
}


# Like W32_GETREG, except prepend HKEY_CURRENT_USER\SOFTWARE and
# HKEY_LOCAL_MACHINE\SOFTWARE to the first argument, returning the first result
# found. Also accounts for the fact that on 64-bit machines, 32-bit software has
# its own area, under SOFTWARE\Wow6432node.
#
local rule software-registry-value ( path : data ? )
{
    local result ;
    for local root in HKEY_CURRENT_USER HKEY_LOCAL_MACHINE
    {
        for local x64elt in "" Wow6432node\\ # Account for 64-bit windows
        {
            if ! $(result)
            {
                result = [ W32_GETREG $(root)\\SOFTWARE\\$(x64elt)$(path) : $(data) ] ;
            }
        }

    }
    return $(result) ;
}


.windows-drive-letter-re = ^([A-Za-z]):[\\/](.*) ;
.cygwin-drive-letter-re = ^/cygdrive/([a-z])/(.*) ;

.working-directory = [ PWD ] ;
.working-drive-letter = [ SUBST $(.working-directory) $(.windows-drive-letter-re) $1 ] ;
.working-drive-letter ?= [ SUBST $(.working-directory) $(.cygwin-drive-letter-re) $1 ] ;


local rule windows-to-cygwin-path ( path )
{
    # If path is rooted with a drive letter, rewrite it using the /cygdrive
    # mountpoint.
    local p = [ SUBST $(path:T) $(.windows-drive-letter-re) /cygdrive/$1/$2 ] ;

    # Else if path is rooted without a drive letter, use the working directory.
    p ?= [ SUBST $(path:T) ^/(.*) /cygdrive/$(.working-drive-letter:L)/$2 ] ;

    # Else return the path unchanged.
    return $(p:E=$(path:T)) ;
}


# :W only works in Cygwin builds of bjam.  This one works on NT builds as well.
#
local rule cygwin-to-windows-path ( path )
{
    path = $(path:R="") ; # strip any trailing slash

    local drive-letter = [ SUBST $(path) $(.cygwin-drive-letter-re) $1:/$2 ] ;
    if $(drive-letter)
    {
        path = $(drive-letter) ;
    }
    else if $(path:R=/x) = $(path) # already rooted?
    {
        # Look for a cygwin mount that includes each head sequence in $(path).
        local head = $(path) ;
        local tail = "" ;

        while $(head)
        {
            local root = [ software-registry-value
                "Cygnus Solutions\\Cygwin\\mounts v2\\"$(head) : native ] ;

            if $(root)
            {
                path = $(tail:R=$(root)) ;
                head = ;
            }
            tail = $(tail:R=$(head:D=)) ;

            if $(head) = /
            {
                head = ;
            }
            else
            {
                head = $(head:D) ;
            }
        }
    }
    return [ regex.replace $(path:R="") / \\ ] ;
}


# Convert a *nix path to native.
#
local rule *nix-path-to-native ( path )
{
    if [ os.name ] = NT
    {
        path = [ cygwin-to-windows-path $(path) ] ;
    }
    return $(path) ;
}


# Convert an NT path to native.
#
local rule windows-path-to-native ( path )
{
    if [ os.name ] = NT
    {
        return $(path) ;
    }
    else
    {
        return [ windows-to-cygwin-path $(path) ] ;
    }
}


# Return nonempty if path looks like a windows path, i.e. it starts with a drive
# letter or contains backslashes.
#
local rule guess-windows-path ( path )
{
    return [ SUBST $(path) ($(.windows-drive-letter-re)|.*([\\]).*) $1 ] ;
}


local rule path-to-native ( paths * )
{
    local result ;

    for local p in $(paths)
    {
        if [ guess-windows-path $(p) ]
        {
            result += [ windows-path-to-native $(p) ] ;
        }
        else
        {
            result += [ *nix-path-to-native $(p:T) ] ;
        }
    }
    return $(result) ;
}


# Validate the version string and extract the major/minor part we care about.
#
local rule split-version ( version )
{
    local major-minor = [ MATCH ^([0-9]+)\.([0-9]+)(.*)$ : $(version) : 1 2 3 ] ;
    if ! $(major-minor[2]) || $(major-minor[3])
    {
        ECHO "Warning: \"using python\" expects a two part (major, minor) version number; got" $(version) instead ;

        # Add a zero to account for the missing digit if necessary.
        major-minor += 0 ;
    }

    return $(major-minor[1]) $(major-minor[2]) ;
}


# Build a list of versions from 3.4 down to 1.5. Because bjam can not enumerate
# registry sub-keys, we have no way of finding a version with a 2-digit minor
# version, e.g. 2.10 -- let us hope that never happens.
#
.version-countdown = ;
for local v in [ numbers.range 15 34 ]
{
    .version-countdown = [ SUBST $(v) (.)(.*) $1.$2 ] $(.version-countdown) ;
}


local rule windows-installed-pythons ( version ? )
{
    version ?= $(.version-countdown) ;
    local interpreters ;

    for local v in $(version)
    {
        local install-path = [
          software-registry-value "Python\\PythonCore\\"$(v)"\\InstallPath" ] ;

        if $(install-path)
        {
            install-path = [ windows-path-to-native $(install-path) ] ;
            debug-message Registry indicates Python $(v) installed at \"$(install-path)\" ;
        }

        interpreters += $(:E=python:R=$(install-path)) ;
    }
    return $(interpreters) ;
}


local rule darwin-installed-pythons ( version ? )
{
    version ?= $(.version-countdown) ;

    local prefix
      = [ GLOB /System/Library/Frameworks /Library/Frameworks
          : Python.framework ] ;

    return $(prefix)/Versions/$(version)/bin/python ;
}


# Assume "python-cmd" invokes a python interpreter and invoke it to extract all
# the information we care about from its "sys" module. Returns void if
# unsuccessful.
#
local rule probe ( python-cmd )
{
    # Avoid invoking a Cygwin symlink on NT.
    local skip-symlink ;
    if [ os.name ] = NT
    {
        skip-symlink = [ invokes-cygwin-symlink $(python-cmd) ] ;
    }

    if $(skip-symlink)
    {
        debug-message -------------------------------------------------------------------- ;
        debug-message \"$(python-cmd)\" would attempt to invoke a Cygwin symlink, ;
        debug-message causing a bjam built for Windows to hang. ;
        debug-message ;
        debug-message If you intend to target a Cygwin build of Python, please ;
        debug-message replace the path to the link with the path to a real executable ;
        debug-message (guessing: \"$(skip-symlink)\") "in" your 'using python' line ;
        debug-message "in" user-config.jam or site-config.jam. Do not forget to escape ;
        debug-message backslashes ;
        debug-message -------------------------------------------------------------------- ;
    }
    else
    {
        # Prepare a List of Python format strings and expressions that can be
        # used to print the constants we want from the sys module.

        # We do not really want sys.version since that is a complicated string,
        # so get the information from sys.version_info instead.
        local format = "version=%d.%d" ;
        local exprs = "version_info[0]" "version_info[1]" ;

        for local s in $(sys-elements[2-])
        {
            format += $(s)=%s ;
            exprs += $(s) ;
        }

        # Invoke Python and ask it for all those values.
        local full-cmd =
            $(python-cmd)" -c \"from sys import *; print('"$(format:J=\\n)"' % ("$(exprs:J=,)"))\"" ;

        local output = [ shell-cmd $(full-cmd) ] ;
        if $(output)
        {
            # Parse the output to get all the results.
            local nl = "

" ;
            for s in $(sys-elements)
            {
                # These variables are expected to be declared local in the
                # caller, so Jam's dynamic scoping will set their values there.
                sys.$(s) = [ SUBST $(output) \\<$(s)=([^$(nl)]+) $1 ] ;
            }
        }
        return $(output) ;
    }
}


# Make sure the "libraries" and "includes" variables (in an enclosing scope)
# have a value based on the information given.
#
local rule compute-default-paths ( target-os : version ? : prefix ? :
    exec-prefix ? )
{
    exec-prefix ?= $(prefix) ;

    if $(target-os) = windows
    {
        # The exec_prefix is where you're supposed to look for machine-specific
        # libraries.
        local default-library-path = $(exec-prefix)\\libs ;
        local default-include-path = $(:E=Include:R=$(prefix)) ;

        # If the interpreter was found in a directory called "PCBuild" or
        # "PCBuild8," assume we're looking at a Python built from the source
        # distro, and go up one additional level to the default root. Otherwise,
        # the default root is the directory where the interpreter was found.

        # We ask Python itself what the executable path is in case of
        # intermediate symlinks or shell scripts.
        local executable-dir = $(sys.executable:D) ;

        if [ MATCH ^(PCBuild) : $(executable-dir:D=) ]
        {
            debug-message "This Python appears to reside in a source distribution;" ;
            debug-message "prepending \""$(executable-dir)"\" to default library search path" ;

            default-library-path = $(executable-dir) $(default-library-path) ;

            default-include-path = $(:E=PC:R=$(executable-dir:D)) $(default-include-path) ;

            debug-message "and \""$(default-include-path[1])"\" to default #include path" ;
        }

        libraries ?= $(default-library-path) ;
        includes ?= $(default-include-path) ;
    }
    else
    {
        includes ?= $(prefix)/include/python$(version) ;

        local lib = $(exec-prefix)/lib ;
        libraries ?= $(lib)/python$(version)/config $(lib) ;
    }
}

# The version of the python interpreter to use.
feature.feature python : : propagated ;
feature.feature python.interpreter : : free ;

toolset.flags python.capture-output PYTHON : <python.interpreter> ;

#
# Support for Python configured --with-pydebug
#
feature.feature python-debugging : off on : propagated ;
builtin.variant debug-python : debug : <python-debugging>on ;


# Return a list of candidate commands to try when looking for a Python
# interpreter. prefix is expected to be a native path.
#
local rule candidate-interpreters ( version ? : prefix ? : target-os )
{
    local bin-path = bin ;
    if $(target-os) = windows
    {
        # On Windows, look in the root directory itself and, to work with the
        # result of a build-from-source, the PCBuild directory.
        bin-path = PCBuild8 PCBuild "" ;
    }

    bin-path = $(bin-path:R=$(prefix)) ;

    if $(target-os) in windows darwin
    {
        return                                            # Search:
            $(:E=python:R=$(bin-path))                    #   Relative to the prefix, if any
            python                                        #   In the PATH
            [ $(target-os)-installed-pythons $(version) ] #   Standard install locations
        ;
    }
    else
    {
        # Search relative to the prefix, or if none supplied, in PATH.
        local unversioned = $(:E=python:R=$(bin-path:E=)) ;

        # If a version was specified, look for a python with that specific
        # version appended before looking for one called, simply, "python"
        return $(unversioned)$(version) $(unversioned) ;
    }
}


# Compute system library dependencies for targets linking with static Python
# libraries.
#
# On many systems, Python uses libraries such as pthreads or libdl. Since static
# libraries carry no library dependency information of their own that the linker
# can extract, these extra dependencies have to be given explicitly on the link
# line of the client.  The information about these dependencies is packaged into
# the "python" target below.
#
# Even where Python itself uses pthreads, it never allows extension modules to
# be entered concurrently (unless they explicitly give up the interpreter lock).
# Therefore, extension modules do not need the efficiency overhead of threadsafe
# code as produced by <threading>multi, and we handle libpthread along with
# other libraries here. Note: this optimization is based on an assumption that
# the compiler generates link-compatible code in both the single- and
# multi-threaded cases, and that system libraries do not change their ABIs
# either.
#
# Returns a list of usage-requirements that link to the necessary system
# libraries.
#
local rule system-library-dependencies ( target-os )
{
    switch $(target-os)
    {
        case s[uo][nl]* : # solaris, sun, sunos
            # Add a librt dependency for the gcc toolset on SunOS (the sun
            # toolset adds -lrt unconditionally). While this appears to
            # duplicate the logic already in gcc.jam, it does not as long as
            # we are not forcing <threading>multi.

            # On solaris 10, distutils.sysconfig.get_config_var('LIBS') yields
            # '-lresolv -lsocket -lnsl -lrt -ldl'. However, that does not seem
            # to be the right list for extension modules. For example, on my
            # installation, adding -ldl causes at least one test to fail because
            # the library can not be found and removing it causes no failures.

            # Apparently, though, we need to add -lrt for gcc.
            return <toolset>gcc:<library>rt ;

        case osf : return  <library>pthread <toolset>gcc:<library>rt ;

        case qnx* : return ;
        case darwin : return ;
        case windows : return ;
        case haiku : return ;

        case hpux : return  <library>rt ;
        case *bsd : return  <library>pthread <toolset>gcc:<library>util ;

        case aix : return  <library>pthread <library>dl ;

        case * : return  <library>pthread <library>dl
            <toolset>gcc:<library>util <toolset-intel:platform>linux:<library>util ;
    }
}


# Declare a target to represent Python's library.
#
local rule declare-libpython-target ( version ? : requirements * )
{
    # Compute the representation of Python version in the name of Python's
    # library file.
    local lib-version = $(version) ;
    if <target-os>windows in $(requirements)
    {
        local major-minor = [ split-version $(version) ] ;
        lib-version = $(major-minor:J="") ;
        if <python-debugging>on in $(requirements)
        {
            lib-version = $(lib-version)_d ;
        }
    }

    if ! $(lib-version)
    {
        ECHO *** warning: could not determine Python version, which will ;
        ECHO *** warning: probably prevent us from linking with the python ;
        ECHO *** warning: library.  Consider explicitly passing the version ;
        ECHO *** warning: to 'using python'. ;
    }

    # Declare it.
    lib python.lib : : <name>python$(lib-version) $(requirements) ;
}


# Implementation of init.
local rule configure ( version ? : cmd-or-prefix ? : includes * : libraries ? :
    condition * : extension-suffix ? )
{
    local prefix ;
    local exec-prefix ;
    local cmds-to-try ;
    local interpreter-cmd ;

    local target-os = [ feature.get-values target-os : $(condition) ] ;
    target-os ?= [ feature.defaults target-os ] ;
    target-os = $(target-os:G=) ;

    if $(target-os) = windows && <python-debugging>on in $(condition)
    {
        extension-suffix ?= _d ;
    }
    extension-suffix ?= "" ;

    # Normalize and dissect any version number.
    local major-minor ;
    if $(version)
    {
        major-minor = [ split-version $(version) ] ;
        version = $(major-minor:J=.) ;
    }

    local cmds-to-try ;

    if ! $(cmd-or-prefix) || [ GLOB $(cmd-or-prefix) : * ]
    {
        # If the user did not pass a command, whatever we got was a prefix.
        prefix = $(cmd-or-prefix) ;
        cmds-to-try = [ candidate-interpreters $(version) : $(prefix) : $(target-os) ] ;
    }
    else
    {
        # Work with the command the user gave us.
        cmds-to-try = $(cmd-or-prefix) ;

        # On Windows, do not nail down the interpreter command just yet in case
        # the user specified something that turns out to be a cygwin symlink,
        # which could bring down bjam if we invoke it.
        if $(target-os) != windows
        {
            interpreter-cmd = $(cmd-or-prefix) ;
        }
    }

    # Values to use in case we can not really find anything in the system.
    local fallback-cmd = $(cmds-to-try[1]) ;
    local fallback-version ;

    # Anything left to find or check?
    if ! ( $(interpreter-cmd) && $(includes) && $(libraries) )
    {
        # Values to be extracted from python's sys module. These will be set by
        # the probe rule, above, using Jam's dynamic scoping.
        local sys-elements = version platform prefix exec_prefix executable ;
        local sys.$(sys-elements) ;

        # Compute the string Python's sys.platform needs to match. If not
        # targeting Windows or cygwin we will assume only native builds can
        # possibly run, so we will not require a match and we leave sys.platform
        # blank.
        local platform ;
        switch $(target-os)
        {
            case windows : platform = win32 ;
            case cygwin : platform = cygwin ;
        }

        while $(cmds-to-try)
        {
            # Pop top command.
            local cmd = $(cmds-to-try[1]) ;
            cmds-to-try = $(cmds-to-try[2-]) ;

            debug-message Checking interpreter command \"$(cmd)\"... ;
            if [ probe $(cmd) ]
            {
                fallback-version ?= $(sys.version) ;

                # Check for version/platform validity.
                for local x in version platform
                {
                    if $($(x)) && $($(x)) != $(sys.$(x))
                    {
                        debug-message ...$(x) "mismatch (looking for"
                            $($(x)) but found $(sys.$(x))")" ;
                        cmd = ;
                    }
                }

                if $(cmd)
                {
                    debug-message ...requested configuration matched! ;

                    exec-prefix = $(sys.exec_prefix) ;

                    compute-default-paths $(target-os) : $(sys.version) :
                        $(sys.prefix) : $(sys.exec_prefix) ;

                    version = $(sys.version) ;
                    interpreter-cmd ?= $(cmd) ;
                    cmds-to-try = ;  # All done.
                }
            }
            else
            {
                debug-message ...does not invoke a working interpreter ;
            }
        }
    }

    # Anything left to compute?
    if $(includes) && $(libraries)
    {
        .configured = true ;
    }
    else
    {
        version ?= $(fallback-version) ;
        version ?= 2.5 ;
        exec-prefix ?= $(prefix) ;
        compute-default-paths $(target-os) : $(version) : $(prefix:E=) ;
    }

    if ! $(interpreter-cmd)
    {
        fallback-cmd ?= python ;
        debug-message No working Python interpreter found. ;
        if [ os.name ] != NT || ! [ invokes-cygwin-symlink $(fallback-cmd) ]
        {
            interpreter-cmd = $(fallback-cmd) ;
            debug-message falling back to \"$(interpreter-cmd)\" ;
        }
    }

    includes = [ path-to-native $(includes) ] ;
    libraries = [ path-to-native $(libraries) ] ;

    debug-message "Details of this Python configuration:" ;
    debug-message "  interpreter command:" \"$(interpreter-cmd:E=<empty>)\" ;
    debug-message "  include path:" \"$(includes:E=<empty>)\" ;
    debug-message "  library path:" \"$(libraries:E=<empty>)\" ;
    if $(target-os) = windows
    {
        debug-message "  DLL search path:" \"$(exec-prefix:E=<empty>)\" ;
    }

    #
    # End autoconfiguration sequence.
    #
    local target-requirements = $(condition) ;

    # Add the version, if any, to the target requirements.
    if $(version)
    {
        if ! $(version) in [ feature.values python ]
        {
            feature.extend python : $(version) ;
        }
        target-requirements += <python>$(version:E=default) ;
    }

    target-requirements += <target-os>$(target-os) ;

    # See if we can find a framework directory on darwin.
    local framework-directory ;
    if $(target-os) = darwin
    {
        # Search upward for the framework directory.
        local framework-directory = $(libraries[-1]) ;
        while $(framework-directory:D=) && $(framework-directory:D=) != Python.framework
        {
            framework-directory = $(framework-directory:D) ;
        }

        if $(framework-directory:D=) = Python.framework
        {
            debug-message framework directory is \"$(framework-directory)\" ;
        }
        else
        {
            debug-message "no framework directory found; using library path" ;
            framework-directory = ;
        }
    }

    local dll-path = $(libraries) ;

    # Make sure that we can find the Python DLL on Windows.
    if ( $(target-os) = windows ) && $(exec-prefix)
    {
        dll-path += $(exec-prefix) ;
    }

    #
    # Prepare usage requirements.
    #
    local usage-requirements = [ system-library-dependencies $(target-os) ] ;
    usage-requirements += <include>$(includes) <python.interpreter>$(interpreter-cmd) ;
    if <python-debugging>on in $(condition)
    {
        if $(target-os) = windows
        {
            # In pyconfig.h, Py_DEBUG is set if _DEBUG is set. If we define
            # Py_DEBUG we will get multiple definition warnings.
            usage-requirements += <define>_DEBUG ;
        }
        else
        {
            usage-requirements += <define>Py_DEBUG ;
        }
    }

    # Global, but conditional, requirements to give access to the interpreter
    # for general utilities, like other toolsets, that run Python scripts.
    toolset.add-requirements
        $(target-requirements:J=,):<python.interpreter>$(interpreter-cmd) ;

    # Register the right suffix for extensions.
    register-extension-suffix $(extension-suffix) : $(target-requirements) ;

    #
    # Declare the "python" target. This should really be called
    # python_for_embedding.
    #

    if $(framework-directory)
    {
        alias python
          :
          : $(target-requirements)
          :
          : $(usage-requirements) <framework>$(framework-directory)
          ;
    }
    else
    {
        declare-libpython-target $(version) : $(target-requirements) ;

        # This is an evil hack.  On, Windows, when Python is embedded, nothing
        # seems to set up sys.path to include Python's standard library
        # (http://article.gmane.org/gmane.comp.python.general/544986). The evil
        # here, aside from the workaround necessitated by Python's bug, is that:
        #
        # a. we're guessing the location of the python standard library from the
        #    location of pythonXX.lib
        #
        # b. we're hijacking the <testing.launcher> property to get the
        #    environment variable set up, and the user may want to use it for
        #    something else (e.g. launch the debugger).
        local set-PYTHONPATH ;
        if $(target-os) = windows
        {
            set-PYTHONPATH = [ common.prepend-path-variable-command PYTHONPATH :
                $(libraries:D)/Lib ] ;
        }

        alias python
          :
          : $(target-requirements)
          :
            # Why python.lib must be listed here instead of along with the
            # system libs is a mystery, but if we do not do it, on cygwin,
            # -lpythonX.Y never appears in the command line (although it does on
            # linux).
          : $(usage-requirements)
            <testing.launcher>$(set-PYTHONPATH)
              <library-path>$(libraries) <dll-path>$(dll-path) <library>python.lib
          ;
    }

    # On *nix, we do not want to link either Boost.Python or Python extensions
    # to libpython, because the Python interpreter itself provides all those
    # symbols. If we linked to libpython, we would get duplicate symbols. So
    # declare two targets -- one for building extensions and another for
    # embedding.
    if $(target-os) in windows cygwin
    {
        alias python_for_extensions : python : $(target-requirements) ;
    }
    else if $(target-os) = darwin {
        alias python_for_extensions
            :
            : $(target-requirements)
            :
            : $(usage-requirements) <linkflags>"-undefined dynamic_lookup"
            ;
    }
    # On AIX we need Python extensions and Boost.Python to import symbols from
    # the Python interpreter. Dynamic libraries opened with dlopen() do not
    # inherit the symbols from the Python interpreter.
    else if $(target-os) = aix
    {
        alias python_for_extensions
            :
            : $(target-requirements)
            :
            : $(usage-requirements) <linkflags>-Wl,-bI:$(libraries[1])/python.exp
            ;
    }
    else
    {
        alias python_for_extensions
            :
            : $(target-requirements)
            :
            : $(usage-requirements)
            ;
    }
}


rule configured ( )
{
     return $(.configured) ;
}


type.register PYTHON_EXTENSION : : SHARED_LIB ;


local rule register-extension-suffix ( root : condition * )
{
    local suffix ;

    switch [ feature.get-values target-os : $(condition) ]
    {
        case windows : suffix = pyd ;
        case cygwin : suffix = dll ;
        case hpux :
        {
            if [ feature.get-values python : $(condition) ] in 1.5 1.6 2.0 2.1 2.2 2.3 2.4
            {
                suffix = sl ;
            }
            else
            {
                suffix = so ;
            }
        }
        case * : suffix = so ;
    }

    type.set-generated-target-suffix PYTHON_EXTENSION : $(condition) : <$(root).$(suffix)> ;
}


# Unset 'lib' prefix for PYTHON_EXTENSION
type.set-generated-target-prefix PYTHON_EXTENSION : : "" ;


rule python-extension ( name : sources * : requirements * : default-build * :
                        usage-requirements * )
{
    if [ configured ]
    {
        requirements += <use>/python//python_for_extensions ;
    }
    requirements += <suppress-import-lib>true ;

    local project = [ project.current ] ;

    targets.main-target-alternative
        [ new typed-target $(name) : $(project) : PYTHON_EXTENSION
            : [ targets.main-target-sources $(sources) : $(name) ]
            : [ targets.main-target-requirements $(requirements) : $(project) ]
            : [ targets.main-target-default-build $(default-build) : $(project) ]
        ] ;
}

IMPORT python : python-extension : : python-extension ;

rule py2to3
{
    common.copy $(<) : $(>) ;
    2to3 $(<) ;
}

actions 2to3
{
    2to3 -wn --no-diffs "$(<)"
    2to3 -dwn --no-diffs "$(<)"
}


# Support for testing.
type.register PY : py ;
type.register RUN_PYD_OUTPUT ;
type.register RUN_PYD : : TEST ;


class python-test-generator : generator
{
    import set ;

    rule __init__ ( * : * )
    {
        generator.__init__ $(1) : $(2) : $(3) : $(4) : $(5) : $(6) : $(7) : $(8) : $(9) ;
        self.composing = true ;
    }

    rule run ( project name ? : property-set : sources * : multiple ? )
    {
        local pyversion = [ $(property-set).get <python> ] ;
        local python ;
        local other-pythons ;

        # Make new target that converting Python source by 2to3 when running with Python 3.
        local rule make-2to3-source ( source )
        {
            if $(pyversion) >= 3.0
            {
                local a = [ new action $(source) : python.py2to3 : $(property-set) ] ;
                local t =  [ utility.basename [ $(s).name ] ] ;
                local p = [ new file-target $(t) : PY : $(project) : $(a) ] ;
                return $(p) ;
            }
            else
            {
                return $(source) ;
            }
        }

        for local s in $(sources)
        {
            if [ $(s).type ] = PY
            {
                if ! $(python)
                {
                    # First Python source ends up on command line.
                    python = [ make-2to3-source $(s) ] ;

                }
                else
                {
                    # Other Python sources become dependencies.
                    other-pythons += [ make-2to3-source $(s) ] ;
                }
            }
        }

        local extensions ;
        for local s in $(sources)
        {
            if [ $(s).type ] = PYTHON_EXTENSION
            {
                extensions += $(s) ;
            }
        }

        local libs ;
        for local s in $(sources)
        {
            if [ type.is-derived [ $(s).type ] LIB ]
              && ! $(s) in $(extensions)
            {
                libs += $(s) ;
            }
        }

        local new-sources ;
        for local s in $(sources)
        {
            if [ type.is-derived [ $(s).type ] CPP ]
            {
                local name = [ utility.basename [ $(s).name ] ] ;
                if $(name) = [ utility.basename [ $(python).name ] ]
                {
                    name = $(name)_ext ;
                }
                local extension = [ generators.construct $(project) $(name) :
                  PYTHON_EXTENSION : $(property-set) : $(s) $(libs) ] ;

                # The important part of usage requirements returned from
                # PYTHON_EXTENSION generator are xdll-path properties that will
                # allow us to find the python extension at runtime.
                property-set = [ $(property-set).add $(extension[1]) ] ;

                # Ignore usage requirements. We're a top-level generator and
                # nobody is going to use what we generate.
                new-sources += $(extension[2-]) ;
            }
        }

        property-set = [ $(property-set).add-raw <dependency>$(other-pythons) ] ;

        return [ construct-result $(python) $(extensions) $(new-sources) :
            $(project) $(name) : $(property-set) ] ;
    }
}


generators.register
  [ new python-test-generator python.capture-output : : RUN_PYD_OUTPUT ] ;

generators.register-standard testing.expect-success
  : RUN_PYD_OUTPUT : RUN_PYD ;


# There are two different ways of spelling OS names. One is used for [ os.name ]
# and the other is used for the <host-os> and <target-os> properties. Until that
# is remedied, this sets up a crude mapping from the latter to the former, that
# will work *for the purposes of cygwin/NT cross-builds only*. Could not think
# of a better name than "translate".
#
.translate-os-windows = NT ;
.translate-os-cygwin = CYGWIN ;
local rule translate-os ( src-os )
{
    local x = $(.translate-os-$(src-os)) [ os.name ] ;
    return $(x[1]) ;
}


# Extract the path to a single ".pyd" source. This is used to build the
# PYTHONPATH for running bpl tests.
#
local rule pyd-pythonpath ( source )
{
    return [ on $(source) return $(LOCATE) $(SEARCH) ] ;
}


# The flag settings on testing.capture-output do not apply to python.capture
# output at the moment. Redo this explicitly.
toolset.flags python.capture-output ARGS <testing.arg> ;


rule capture-output ( target : sources * : properties * )
{
    # Setup up a proper DLL search path. Here, $(sources[1]) is a python module
    # and $(sources[2]) is a DLL. Only $(sources[1]) is passed to
    # testing.capture-output, so RUN_PATH variable on $(sources[2]) is not
    # consulted. Move it over explicitly.
    RUN_PATH on $(sources[1]) = [ on $(sources[2-]) return $(RUN_PATH) ] ;

    PYTHONPATH  = [ sequence.transform pyd-pythonpath : $(sources[2-]) ] ;
    PYTHONPATH += [ feature.get-values pythonpath : $(properties) ] ;

    # After test is run, we remove the Python module, but not the Python script.
    testing.capture-output $(target) : $(sources[1]) : $(properties) :
        $(sources[2-]) ;

    # PYTHONPATH is different; it will be interpreted by whichever Python is
    # invoked and so must follow path rules for the target os. The only OSes
    # where we can run python for other OSes currently are NT and CYGWIN so we
    # only need to handle those cases.
    local target-os = [ feature.get-values target-os : $(properties) ] ;
    # Oddly, host-os is not in properties, so grab the default value.
    local host-os = [ feature.defaults host-os ] ;
    host-os = $(host-os:G=) ;
    if $(target-os) != $(host-os) && $(target-os) in windows cygwin && $(host-os) in windows cygwin
    {
        PYTHONPATH = [ sequence.transform $(host-os)-to-$(target-os)-path :
            $(PYTHONPATH) ] ;
    }
    local path-separator = [ os.path-separator [ translate-os $(target-os) ] ] ;
    local set-PYTHONPATH = [ common.variable-setting-command PYTHONPATH :
        $(PYTHONPATH:J=$(path-separator)) ] ;
    LAUNCHER on $(target) = $(set-PYTHONPATH) [ on $(target) return \"$(PYTHON)\" ] ;
}


rule bpl-test ( name : sources * : requirements * )
{
    local s ;
    sources ?= $(name).py $(name).cpp ;
    return [ testing.make-test run-pyd : $(sources) /boost/python//boost_python
        : $(requirements) : $(name) ] ;
}


IMPORT $(__name__) : bpl-test : : bpl-test ;