Doc/library/traceback.rst

:mod:`traceback` --- Print or retrieve a stack traceback
========================================================

.. module:: traceback
   :synopsis: Print or retrieve a stack traceback.


This module provides a standard interface to extract, format and print stack
traces of Python programs.  It exactly mimics the behavior of the Python
interpreter when it prints a stack trace.  This is useful when you want to print
stack traces under program control, such as in a "wrapper" around the
interpreter.

.. index:: object: traceback

The module uses traceback objects --- this is the object type that is stored in
the variables :data:`sys.exc_traceback` (deprecated) and
:data:`sys.last_traceback` and returned as the third item from
:func:`sys.exc_info`.

The module defines the following functions:


.. function:: print_tb(tb[, limit[, file]])

   Print up to *limit* stack trace entries from the traceback object *tb*. If
   *limit* is omitted or ``None``, all entries are printed. If *file* is omitted
   or ``None``, the output goes to ``sys.stderr``; otherwise it should be an
   open file or file-like object to receive the output.


.. function:: print_exception(etype, value, tb[, limit[, file]])

   Print exception information and up to *limit* stack trace entries from the
   traceback *tb* to *file*. This differs from :func:`print_tb` in the following
   ways: (1) if *tb* is not ``None``, it prints a header ``Traceback (most
   recent call last):``; (2) it prints the exception *etype* and *value* after
   the stack trace; (3) if *etype* is :exc:`SyntaxError` and *value* has the
   appropriate format, it prints the line where the syntax error occurred with a
   caret indicating the approximate position of the error.


.. function:: print_exc([limit[, file]])

   This is a shorthand for ``print_exception(sys.exc_type, sys.exc_value,
   sys.exc_traceback, limit, file)``.  (In fact, it uses :func:`sys.exc_info` to
   retrieve the same information in a thread-safe way instead of using the
   deprecated variables.)


.. function:: format_exc([limit])

   This is like ``print_exc(limit)`` but returns a string instead of printing to
   a file.

   .. versionadded:: 2.4


.. function:: print_last([limit[, file]])

   This is a shorthand for ``print_exception(sys.last_type, sys.last_value,
   sys.last_traceback, limit, file)``.  In general it will work only after
   an exception has reached an interactive prompt (see :data:`sys.last_type`).


.. function:: print_stack([f[, limit[, file]]])

   This function prints a stack trace from its invocation point. The optional
   *f* argument can be used to specify an alternate stack frame to start. The
   optional limit* and *file* arguments have the same meaning as for
   :func:`print_exception`.


.. function:: extract_tb(tb[, limit])

   Return a list of up to *limit* "pre-processed" stack trace entries extracted
   from the traceback object *tb*.  It is useful for alternate formatting of
   stack traces.  If *limit* is omitted or ``None``, all entries are extracted.
   A "pre-processed" stack trace entry is a 4-tuple (*filename*, *line number*,
   function name*, *text*) representing the information that is usually printed
   for a stack trace.  The *text* is a string with leading and trailing
   whitespace stripped; if the source is not available it is ``None``.


.. function:: extract_stack([f[, limit]])

   Extract the raw traceback from the current stack frame.  The return value has
   the same format as for :func:`extract_tb`.  The optional *f* and *limit*
   arguments have the same meaning as for :func:`print_stack`.


.. function:: format_list(extracted_list)

   Given a list of tuples as returned by :func:`extract_tb` or
   :func:`extract_stack`, return a list of strings ready for printing.  Each
   string in the resulting list corresponds to the item with the same index in
   the argument list.  Each string ends in a newline; the strings may contain
   internal newlines as well, for those items whose source text line is not
   ``None``.


.. function:: format_exception_only(etype, value)

   Format the exception part of a traceback.  The arguments are the exception
   type, *etype* and *value* such as given by ``sys.last_type`` and
   ``sys.last_value``.  The return value is a list of strings, each ending in a
   newline.  Normally, the list contains a single string; however, for
   :exc:`SyntaxError` exceptions, it contains several lines that (when printed)
   display detailed information about where the syntax error occurred.  The
   message indicating which exception occurred is the always last string in the
   list.


.. function:: format_exception(etype, value, tb[, limit])

   Format a stack trace and the exception information.  The arguments  have the
   same meaning as the corresponding arguments to :func:`print_exception`.  The
   return value is a list of strings, each ending in a newline and some
   containing internal newlines.  When these lines are concatenated and printed,
   exactly the same text is printed as does :func:`print_exception`.


.. function:: format_tb(tb[, limit])

   A shorthand for ``format_list(extract_tb(tb, limit))``.


.. function:: format_stack([f[, limit]])

   A shorthand for ``format_list(extract_stack(f, limit))``.


.. function:: tb_lineno(tb)

   This function returns the current line number set in the traceback object.
   This function was necessary because in versions of Python prior to 2.3 when
   the :option:`-O` flag was passed to Python the ``tb.tb_lineno`` was not
   updated correctly.  This function has no use in versions past 2.3.


.. _traceback-example:

Traceback Examples
------------------

This simple example implements a basic read-eval-print loop, similar to (but
less useful than) the standard Python interactive interpreter loop.  For a more
complete implementation of the interpreter loop, refer to the :mod:`code`
module. ::

   import sys, traceback

   def run_user_code(envdir):
       source = raw_input(">>> ")
       try:
           exec source in envdir
       except:
           print "Exception in user code:"
           print '-'*60
           traceback.print_exc(file=sys.stdout)
           print '-'*60

   envdir = {}
   while 1:
       run_user_code(envdir)


The following example demonstrates the different ways to print and format the
exception and traceback::

   import sys, traceback

   def lumberjack():
       bright_side_of_death()

   def bright_side_of_death():
       return tuple()[0]

   try:
       lumberjack()
   except IndexError:
       exc_type, exc_value, exc_traceback = sys.exc_info()
       print "*** print_tb:"
       traceback.print_tb(exc_traceback, limit=1, file=sys.stdout)
       print "*** print_exception:"
       traceback.print_exception(exc_type, exc_value, exc_traceback,
                                 limit=2, file=sys.stdout)
       print "*** print_exc:"
       traceback.print_exc()
       print "*** format_exc, first and last line:"
       formatted_lines = traceback.format_exc().splitlines()
       print formatted_lines[0]
       print formatted_lines[-1]
       print "*** format_exception:"
       print repr(traceback.format_exception(exc_type, exc_value,
                                             exc_traceback))
       print "*** extract_tb:"
       print repr(traceback.extract_tb(exc_traceback))
       print "*** format_tb:"
       print repr(traceback.format_tb(exc_traceback))
       print "*** tb_lineno:", exc_traceback.tb_lineno


The output for the example would look similar to this::

   *** print_tb:
     File "<doctest...>", line 10, in <module>
       lumberjack()
   *** print_exception:
   Traceback (most recent call last):
     File "<doctest...>", line 10, in <module>
       lumberjack()
     File "<doctest...>", line 4, in lumberjack
       bright_side_of_death()
   IndexError: tuple index out of range
   *** print_exc:
   Traceback (most recent call last):
     File "<doctest...>", line 10, in <module>
       lumberjack()
     File "<doctest...>", line 4, in lumberjack
       bright_side_of_death()
   IndexError: tuple index out of range
   *** format_exc, first and last line:
   Traceback (most recent call last):
   IndexError: tuple index out of range
   *** format_exception:
   ['Traceback (most recent call last):\n',
    '  File "<doctest...>", line 10, in <module>\n    lumberjack()\n',
    '  File "<doctest...>", line 4, in lumberjack\n    bright_side_of_death()\n',
    '  File "<doctest...>", line 7, in bright_side_of_death\n    return tuple()[0]\n',
    'IndexError: tuple index out of range\n']
   *** extract_tb:
   [('<doctest...>', 10, '<module>', 'lumberjack()'),
    ('<doctest...>', 4, 'lumberjack', 'bright_side_of_death()'),
    ('<doctest...>', 7, 'bright_side_of_death', 'return tuple()[0]')]
   *** format_tb:
   ['  File "<doctest...>", line 10, in <module>\n    lumberjack()\n',
    '  File "<doctest...>", line 4, in lumberjack\n    bright_side_of_death()\n',
    '  File "<doctest...>", line 7, in bright_side_of_death\n    return tuple()[0]\n']
   *** tb_lineno: 10


The following example shows the different ways to print and format the stack::

   >>> import traceback
   >>> def another_function():
   ...     lumberstack()
   ...
   >>> def lumberstack():
   ...     traceback.print_stack()
   ...     print repr(traceback.extract_stack())
   ...     print repr(traceback.format_stack())
   ...
   >>> another_function()
     File "<doctest>", line 10, in <module>
       another_function()
     File "<doctest>", line 3, in another_function
       lumberstack()
     File "<doctest>", line 6, in lumberstack
       traceback.print_stack()
   [('<doctest>', 10, '<module>', 'another_function()'),
    ('<doctest>', 3, 'another_function', 'lumberstack()'),
    ('<doctest>', 7, 'lumberstack', 'print repr(traceback.extract_stack())')]
   ['  File "<doctest>", line 10, in <module>\n    another_function()\n',
    '  File "<doctest>", line 3, in another_function\n    lumberstack()\n',
    '  File "<doctest>", line 8, in lumberstack\n    print repr(traceback.format_stack())\n']


This last example demonstrates the final few formatting functions:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> import traceback
   >>> traceback.format_list([('spam.py', 3, '<module>', 'spam.eggs()'),
   ...                        ('eggs.py', 42, 'eggs', 'return "bacon"')])
   ['  File "spam.py", line 3, in <module>\n    spam.eggs()\n',
    '  File "eggs.py", line 42, in eggs\n    return "bacon"\n']
   >>> an_error = IndexError('tuple index out of range')
   >>> traceback.format_exception_only(type(an_error), an_error)
   ['IndexError: tuple index out of range\n']