path: root/Doc/library/traceback.rst

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

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

**Source code:** :source:`Lib/traceback.py`

--------------

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 :data:`sys.last_traceback` variable and returned as the third item from
:func:`sys.exc_info`.

The module defines the following functions:


.. function:: print_tb(tb, limit=None, file=None)

   Print up to *limit* stack trace entries from traceback object *tb* (starting
   from the caller's frame) if *limit* is positive.  Otherwise, print the last
   ``abs(limit)`` entries.  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.

   .. versionchanged:: 3.5
       Added negative *limit* support.


.. function:: print_exception(etype, value, tb, limit=None, file=None, chain=True)

   Print exception information and stack trace entries from traceback object
   *tb* to *file*. This differs from :func:`print_tb` in the following
   ways:

   * if *tb* is not ``None``, it prints a header ``Traceback (most recent
     call last):``
   * it prints the exception *etype* and *value* after the stack trace
   * 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.

   The optional *limit* argument has the same meaning as for :func:`print_tb`.
   If *chain* is true (the default), then chained exceptions (the
   :attr:`__cause__` or :attr:`__context__` attributes of the exception) will be
   printed as well, like the interpreter itself does when printing an unhandled
   exception.


.. function:: print_exc(limit=None, file=None, chain=True)

   This is a shorthand for ``print_exception(*sys.exc_info(), limit, file,
   chain)``.


.. function:: print_last(limit=None, file=None, chain=True)

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


.. function:: print_stack(f=None, limit=None, file=None)

   Print up to *limit* stack trace entries (starting from the invocation
   point) if *limit* is positive.  Otherwise, print the last ``abs(limit)``
   entries.  If *limit* is omitted or ``None``, all entries are printed.
   The optional *f* argument can be used to specify an alternate stack frame
   to start.  The optional *file* argument has the same meaning as for
   :func:`print_tb`.

   .. versionchanged:: 3.5
          Added negative *limit* support.


.. function:: extract_tb(tb, limit=None)

   Return a list of "pre-processed" stack trace entries extracted from the
   traceback object *tb*.  It is useful for alternate formatting of
   stack traces.  The optional *limit* argument has the same meaning as for
   :func:`print_tb`.  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=None, limit=None)

   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 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=None, chain=True)

   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_exc(limit=None, chain=True)

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


.. function:: format_tb(tb, limit=None)

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


.. function:: format_stack(f=None, limit=None)

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

.. function:: clear_frames(tb)

   Clears the local variables of all the stack frames in a traceback *tb*
   by calling the :meth:`clear` method of each frame object.

   .. versionadded:: 3.4

.. function:: walk_stack(f)

   Walk a stack following ``f.f_back`` from the given frame, yielding the frame
   and line number for each frame. If *f* is ``None``, the current stack is
   used. This helper is used with :meth:`StackSummary.extract`.

   .. versionadded:: 3.5

.. function:: walk_tb(tb)

   Walk a traceback following ``tb_next`` yielding the frame and line number
   for each frame. This helper is used with :meth:`StackSummary.extract`.

   .. versionadded:: 3.5

The module also defines the following classes:

:class:`TracebackException` Objects
-----------------------------------

.. versionadded:: 3.5

:class:`TracebackException` objects are created from actual exceptions to
capture data for later printing in a lightweight fashion.

.. class:: TracebackException(exc_type, exc_value, exc_traceback, *, limit=None, lookup_lines=True, capture_locals=False)

   Capture an exception for later rendering. *limit*, *lookup_lines* and
   *capture_locals* are as for the :class:`StackSummary` class.

   Note that when locals are captured, they are also shown in the traceback.

   .. attribute:: __cause__

      A :class:`TracebackException` of the original ``__cause__``.

   .. attribute:: __context__

      A :class:`TracebackException` of the original ``__context__``.

   .. attribute:: __suppress_context__

      The ``__suppress_context__`` value from the original exception.

   .. attribute:: stack

      A :class:`StackSummary` representing the traceback.

   .. attribute:: exc_type

      The class of the original traceback.

   .. attribute:: filename

      For syntax errors - the file name where the error occurred.

   .. attribute:: lineno

      For syntax errors - the line number where the error occurred.

   .. attribute:: text

      For syntax errors - the text where the error occurred.

   .. attribute:: offset

      For syntax errors - the offset into the text where the error occurred.

   .. attribute:: msg

      For syntax errors - the compiler error message.

   .. classmethod:: from_exception(exc, *, limit=None, lookup_lines=True, capture_locals=False)

      Capture an exception for later rendering. *limit*, *lookup_lines* and
      *capture_locals* are as for the :class:`StackSummary` class.

      Note that when locals are captured, they are also shown in the traceback.

   .. method:: format(*, chain=True)

      Format the exception.

      If *chain* is not ``True``, ``__cause__`` and ``__context__`` will not
      be formatted.

      The return value is a generator of strings, each ending in a newline and
      some containing internal newlines. :func:`~traceback.print_exception`
      is a wrapper around this method which just prints the lines to a file.

      The message indicating which exception occurred is always the last
      string in the output.

   .. method::  format_exception_only()

      Format the exception part of the traceback.

      The return value is a generator of strings, each ending in a newline.

      Normally, the generator emits a single string; however, for
      :exc:`SyntaxError` exceptions, it emits several lines that (when
      printed) display detailed information about where the syntax
      error occurred.

      The message indicating which exception occurred is always the last
      string in the output.


:class:`StackSummary` Objects
-----------------------------

.. versionadded:: 3.5

:class:`StackSummary` objects represent a call stack ready for formatting.

.. class:: StackSummary

   .. classmethod:: extract(frame_gen, *, limit=None, lookup_lines=True, capture_locals=False)

      Construct a :class:`StackSummary` object from a frame generator (such as
      is returned by :func:`~traceback.walk_stack` or
      :func:`~traceback.walk_tb`).

      If *limit* is supplied, only this many frames are taken from *frame_gen*.
      If *lookup_lines* is ``False``, the returned :class:`FrameSummary`
      objects will not have read their lines in yet, making the cost of
      creating the :class:`StackSummary` cheaper (which may be valuable if it
      may not actually get formatted). If *capture_locals* is ``True`` the
      local variables in each :class:`FrameSummary` are captured as object
      representations.

   .. classmethod:: from_list(a_list)

      Construct a :class:`StackSummary` object from a supplied old-style list
      of tuples. Each tuple should be a 4-tuple with filename, lineno, name,
      line as the elements.


:class:`FrameSummary` Objects
-----------------------------

.. versionadded:: 3.5

:class:`FrameSummary` objects represent a single frame in a traceback.

.. class:: FrameSummary(filename, lineno, name, lookup_line=True, locals=None, line=None)

   Represent a single frame in the traceback or stack that is being formatted
   or printed. It may optionally have a stringified version of the frames
   locals included in it. If *lookup_line* is ``False``, the source code is not
   looked up until the :class:`FrameSummary` has the :attr:`~FrameSummary.line`
   attribute accessed (which also happens when casting it to a tuple).
   :attr:`~FrameSummary.line` may be directly provided, and will prevent line
   lookups happening at all. *locals* is an optional local variable
   dictionary, and if supplied the variable representations are stored in the
   summary for later display.

.. _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 = input(">>> ")
       try:
           exec(source, envdir)
       except Exception:
           print("Exception in user code:")
           print("-"*60)
           traceback.print_exc(file=sys.stdout)
           print("-"*60)

   envdir = {}
   while True:
       run_user_code(envdir)


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

.. testcode::

   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:

.. testoutput::
   :options: +NORMALIZE_WHITESPACE

   *** 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']