Doc/tutorial/errors.rst

.. _tut-errors:

*********************
Errors and Exceptions
*********************

Until now error messages haven't been more than mentioned, but if you have tried
out the examples you have probably seen some.  There are (at least) two
distinguishable kinds of errors: *syntax errors* and *exceptions*.


.. _tut-syntaxerrors:

Syntax Errors
=============

Syntax errors, also known as parsing errors, are perhaps the most common kind of
complaint you get while you are still learning Python::

   >>> while True print('Hello world')
     File "<stdin>", line 1
       while True print('Hello world')
                      ^
   SyntaxError: invalid syntax

The parser repeats the offending line and displays a little 'arrow' pointing at
the earliest point in the line where the error was detected.  The error is
caused by (or at least detected at) the token *preceding* the arrow: in the
example, the error is detected at the function :func:`print`, since a colon
(``':'``) is missing before it.  File name and line number are printed so you
know where to look in case the input came from a script.


.. _tut-exceptions:

Exceptions
==========

Even if a statement or expression is syntactically correct, it may cause an
error when an attempt is made to execute it. Errors detected during execution
are called *exceptions* and are not unconditionally fatal: you will soon learn
how to handle them in Python programs.  Most exceptions are not handled by
programs, however, and result in error messages as shown here::

   >>> 10 * (1/0)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ZeroDivisionError: division by zero
   >>> 4 + spam*3
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   NameError: name 'spam' is not defined
   >>> '2' + 2
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: Can't convert 'int' object to str implicitly

The last line of the error message indicates what happened. Exceptions come in
different types, and the type is printed as part of the message: the types in
the example are :exc:`ZeroDivisionError`, :exc:`NameError` and :exc:`TypeError`.
The string printed as the exception type is the name of the built-in exception
that occurred.  This is true for all built-in exceptions, but need not be true
for user-defined exceptions (although it is a useful convention). Standard
exception names are built-in identifiers (not reserved keywords).

The rest of the line provides detail based on the type of exception and what
caused it.

The preceding part of the error message shows the context where the exception
happened, in the form of a stack traceback. In general it contains a stack
traceback listing source lines; however, it will not display lines read from
standard input.

:ref:`bltin-exceptions` lists the built-in exceptions and their meanings.


.. _tut-handling:

Handling Exceptions
===================

It is possible to write programs that handle selected exceptions. Look at the
following example, which asks the user for input until a valid integer has been
entered, but allows the user to interrupt the program (using :kbd:`Control-C` or
whatever the operating system supports); note that a user-generated interruption
is signalled by raising the :exc:`KeyboardInterrupt` exception. ::

   >>> while True:
   ...     try:
   ...         x = int(input("Please enter a number: "))
   ...         break
   ...     except ValueError:
   ...         print("Oops!  That was no valid number.  Try again...")
   ...

The :keyword:`try` statement works as follows.

* First, the *try clause* (the statement(s) between the :keyword:`try` and
  :keyword:`except` keywords) is executed.

* If no exception occurs, the *except clause* is skipped and execution of the
  :keyword:`try` statement is finished.

* If an exception occurs during execution of the try clause, the rest of the
  clause is skipped.  Then if its type matches the exception named after the
  :keyword:`except` keyword, the except clause is executed, and then execution
  continues after the :keyword:`try` statement.

* If an exception occurs which does not match the exception named in the except
  clause, it is passed on to outer :keyword:`try` statements; if no handler is
  found, it is an *unhandled exception* and execution stops with a message as
  shown above.

A :keyword:`try` statement may have more than one except clause, to specify
handlers for different exceptions.  At most one handler will be executed.
Handlers only handle exceptions that occur in the corresponding try clause, not
in other handlers of the same :keyword:`!try` statement.  An except clause may
name multiple exceptions as a parenthesized tuple, for example::

   ... except (RuntimeError, TypeError, NameError):
   ...     pass

A class in an :keyword:`except` clause is compatible with an exception if it is
the same class or a base class thereof (but not the other way around --- an
except clause listing a derived class is not compatible with a base class).  For
example, the following code will print B, C, D in that order::

   class B(Exception):
       pass

   class C(B):
       pass

   class D(C):
       pass

   for cls in [B, C, D]:
       try:
           raise cls()
       except D:
           print("D")
       except C:
           print("C")
       except B:
           print("B")

Note that if the except clauses were reversed (with ``except B`` first), it
would have printed B, B, B --- the first matching except clause is triggered.

The last except clause may omit the exception name(s), to serve as a wildcard.
Use this with extreme caution, since it is easy to mask a real programming error
in this way!  It can also be used to print an error message and then re-raise
the exception (allowing a caller to handle the exception as well)::

   import sys

   try:
       f = open('myfile.txt')
       s = f.readline()
       i = int(s.strip())
   except OSError as err:
       print("OS error: {0}".format(err))
   except ValueError:
       print("Could not convert data to an integer.")
   except:
       print("Unexpected error:", sys.exc_info()[0])
       raise

The :keyword:`try` ... :keyword:`except` statement has an optional *else
clause*, which, when present, must follow all except clauses.  It is useful for
code that must be executed if the try clause does not raise an exception.  For
example::

   for arg in sys.argv[1:]:
       try:
           f = open(arg, 'r')
       except OSError:
           print('cannot open', arg)
       else:
           print(arg, 'has', len(f.readlines()), 'lines')
           f.close()

The use of the :keyword:`!else` clause is better than adding additional code to
the :keyword:`try` clause because it avoids accidentally catching an exception
that wasn't raised by the code being protected by the :keyword:`!try` ...
:keyword:`!except` statement.

When an exception occurs, it may have an associated value, also known as the
exception's *argument*. The presence and type of the argument depend on the
exception type.

The except clause may specify a variable after the exception name.  The
variable is bound to an exception instance with the arguments stored in
``instance.args``.  For convenience, the exception instance defines
:meth:`__str__` so the arguments can be printed directly without having to
reference ``.args``.  One may also instantiate an exception first before
raising it and add any attributes to it as desired. ::

   >>> try:
   ...     raise Exception('spam', 'eggs')
   ... except Exception as inst:
   ...     print(type(inst))    # the exception instance
   ...     print(inst.args)     # arguments stored in .args
   ...     print(inst)          # __str__ allows args to be printed directly,
   ...                          # but may be overridden in exception subclasses
   ...     x, y = inst.args     # unpack args
   ...     print('x =', x)
   ...     print('y =', y)
   ...
   <class 'Exception'>
   ('spam', 'eggs')
   ('spam', 'eggs')
   x = spam
   y = eggs

If an exception has arguments, they are printed as the last part ('detail') of
the message for unhandled exceptions.

Exception handlers don't just handle exceptions if they occur immediately in the
try clause, but also if they occur inside functions that are called (even
indirectly) in the try clause. For example::

   >>> def this_fails():
   ...     x = 1/0
   ...
   >>> try:
   ...     this_fails()
   ... except ZeroDivisionError as err:
   ...     print('Handling run-time error:', err)
   ...
   Handling run-time error: division by zero


.. _tut-raising:

Raising Exceptions
==================

The :keyword:`raise` statement allows the programmer to force a specified
exception to occur. For example::

   >>> raise NameError('HiThere')
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   NameError: HiThere

The sole argument to :keyword:`raise` indicates the exception to be raised.
This must be either an exception instance or an exception class (a class that
derives from :class:`Exception`).  If an exception class is passed, it will
be implicitly instantiated by calling its constructor with no arguments::

   raise ValueError  # shorthand for 'raise ValueError()'

If you need to determine whether an exception was raised but don't intend to
handle it, a simpler form of the :keyword:`raise` statement allows you to
re-raise the exception::

   >>> try:
   ...     raise NameError('HiThere')
   ... except NameError:
   ...     print('An exception flew by!')
   ...     raise
   ...
   An exception flew by!
   Traceback (most recent call last):
     File "<stdin>", line 2, in <module>
   NameError: HiThere


.. _tut-userexceptions:

User-defined Exceptions
=======================

Programs may name their own exceptions by creating a new exception class (see
:ref:`tut-classes` for more about Python classes).  Exceptions should typically
be derived from the :exc:`Exception` class, either directly or indirectly.

Exception classes can be defined which do anything any other class can do, but
are usually kept simple, often only offering a number of attributes that allow
information about the error to be extracted by handlers for the exception.  When
creating a module that can raise several distinct errors, a common practice is
to create a base class for exceptions defined by that module, and subclass that
to create specific exception classes for different error conditions::

   class Error(Exception):
       """Base class for exceptions in this module."""
       pass

   class InputError(Error):
       """Exception raised for errors in the input.

       Attributes:
           expression -- input expression in which the error occurred
           message -- explanation of the error
       """

       def __init__(self, expression, message):
           self.expression = expression
           self.message = message

   class TransitionError(Error):
       """Raised when an operation attempts a state transition that's not
       allowed.

       Attributes:
           previous -- state at beginning of transition
           next -- attempted new state
           message -- explanation of why the specific transition is not allowed
       """

       def __init__(self, previous, next, message):
           self.previous = previous
           self.next = next
           self.message = message

Most exceptions are defined with names that end in "Error", similar to the
naming of the standard exceptions.

Many standard modules define their own exceptions to report errors that may
occur in functions they define.  More information on classes is presented in
chapter :ref:`tut-classes`.


.. _tut-cleanup:

Defining Clean-up Actions
=========================

The :keyword:`try` statement has another optional clause which is intended to
define clean-up actions that must be executed under all circumstances.  For
example::

   >>> try:
   ...     raise KeyboardInterrupt
   ... finally:
   ...     print('Goodbye, world!')
   ...
   Goodbye, world!
   Traceback (most recent call last):
     File "<stdin>", line 2, in <module>
   KeyboardInterrupt

If a :keyword:`finally` clause is present, the :keyword:`!finally`
clause will execute as the last task before the :keyword:`try`
statement completes. The :keyword:`!finally` clause runs whether or
not the :keyword:`!try` statement produces an exception. The following
points discuss more complex cases when an exception occurs:

* If an exception occurs during execution of the :keyword:`!try`
  clause, the exception may be handled by an :keyword:`except`
  clause. If the exception is not handled by an :keyword:`!except`
  clause, the exception is re-raised after the :keyword:`!finally`
  clause has been executed.

* An exception could occur during execution of an :keyword:`!except`
  or :keyword:`!else` clause. Again, the exception is re-raised after
  the :keyword:`!finally` clause has been executed.

* If the :keyword:`!try` statement reaches a :keyword:`break`,
  :keyword:`continue` or :keyword:`return` statement, the
  :keyword:`!finally` clause will execute just prior to the
  :keyword:`!break`, :keyword:`!continue` or :keyword:`!return`
  statement's execution.

* If a :keyword:`!finally` clause includes a :keyword:`!return`
  statement, the returned value will be the one from the
  :keyword:`!finally` clause's :keyword:`!return` statement, not the
  value from the :keyword:`!try` clause's :keyword:`!return`
  statement.

For example::

   >>> def bool_return():
   ...     try:
   ...         return True
   ...     finally:
   ...         return False
   ...
   >>> bool_return()
   False

A more complicated example::

   >>> def divide(x, y):
   ...     try:
   ...         result = x / y
   ...     except ZeroDivisionError:
   ...         print("division by zero!")
   ...     else:
   ...         print("result is", result)
   ...     finally:
   ...         print("executing finally clause")
   ...
   >>> divide(2, 1)
   result is 2.0
   executing finally clause
   >>> divide(2, 0)
   division by zero!
   executing finally clause
   >>> divide("2", "1")
   executing finally clause
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 3, in divide
   TypeError: unsupported operand type(s) for /: 'str' and 'str'

As you can see, the :keyword:`finally` clause is executed in any event.  The
:exc:`TypeError` raised by dividing two strings is not handled by the
:keyword:`except` clause and therefore re-raised after the :keyword:`!finally`
clause has been executed.

In real world applications, the :keyword:`finally` clause is useful for
releasing external resources (such as files or network connections), regardless
of whether the use of the resource was successful.


.. _tut-cleanup-with:

Predefined Clean-up Actions
===========================

Some objects define standard clean-up actions to be undertaken when the object
is no longer needed, regardless of whether or not the operation using the object
succeeded or failed. Look at the following example, which tries to open a file
and print its contents to the screen. ::

   for line in open("myfile.txt"):
       print(line, end="")

The problem with this code is that it leaves the file open for an indeterminate
amount of time after this part of the code has finished executing.
This is not an issue in simple scripts, but can be a problem for larger
applications. The :keyword:`with` statement allows objects like files to be
used in a way that ensures they are always cleaned up promptly and correctly. ::

   with open("myfile.txt") as f:
       for line in f:
           print(line, end="")

After the statement is executed, the file *f* is always closed, even if a
problem was encountered while processing the lines. Objects which, like files,
provide predefined clean-up actions will indicate this in their documentation.