: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 is more flexible than the interpreter's default traceback display, and therefore makes it possible to configure certain aspects of the output. Finally, it contains a utility for capturing enough information about an exception to print it later, without the need to save a reference to the actual exception. Since exceptions can be the roots of large objects graph, this utility can significantly improve memory management.

.. index:: pair: object; traceback

The module uses :ref:traceback objects <traceback-objects> --- these are objects of type :class:types.TracebackType, which are assigned to the :attr:~BaseException.__traceback__ field of :class:BaseException instances.

.. seealso::

Module :mod:faulthandler Used to dump Python tracebacks explicitly, on a fault, after a timeout, or on a user signal.

Module :mod:pdb Interactive source code debugger for Python programs.

The module's API can be divided into two parts:

• Module-level functions offering basic functionality, which are useful for interactive inspection of exceptions and tracebacks.

• :class:TracebackException class and its helper classes :class:StackSummary and :class:FrameSummary. These offer both more flexibility in the output generated and the ability to store the information necessary for later formatting without holding references to actual exception and traceback objects.

.. versionadded:: 3.13 Output is colorized by default and can be :ref:controlled using environment variables <using-on-controlling-color>.

Module-Level Functions

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

Print up to limit stack trace entries from :ref:traceback object <traceback-objects> 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 :data:sys.stderr; otherwise it should be an open :term:file <file object> or :term:file-like object to receive the output.

.. note::

  The meaning of the *limit* parameter is different than the meaning
  of :const:`sys.tracebacklimit`. A negative *limit* value corresponds to
  a positive value of :const:`!sys.tracebacklimit`, whereas the behaviour of
  a positive *limit* value cannot be achieved with
  :const:`!sys.tracebacklimit`.

.. versionchanged:: 3.5 Added negative limit support.

.. function:: print_exception(exc, /[, value, tb], limit=None,
file=None, chain=True)

Print exception information and stack trace entries from :ref:traceback object <traceback-objects> 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 type and value after the stack trace

.. index:: single: ^ (caret); marker

• if type(value) 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.

Since Python 3.10, instead of passing value and tb, an exception object can be passed as the first argument. If value and tb are provided, the first argument is ignored in order to provide backwards compatibility.

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

.. versionchanged:: 3.5 The etype argument is ignored and inferred from the type of value.

.. versionchanged:: 3.10 The etype parameter has been renamed to exc and is now positional-only.

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

This is a shorthand for print_exception(sys.exception(), limit=limit, file=file, chain=chain).

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

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

.. 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 :ref:stack frame <frame-objects> 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 :class:StackSummary object representing a list of "pre-processed" stack trace entries extracted from the :ref:traceback object <traceback-objects> 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 :class:FrameSummary object containing attributes :attr:~FrameSummary.filename, :attr:~FrameSummary.lineno, :attr:~FrameSummary.name, and :attr:~FrameSummary.line representing the information that is usually printed for a stack trace.

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

Extract the raw traceback from the current :ref:stack frame <frame-objects>. 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:: print_list(extracted_list, file=None)

Print the list of tuples as returned by :func:extract_tb or :func:extract_stack as a formatted stack trace to the given file. If file is None, the output is written to :data:sys.stderr.

.. function:: format_list(extracted_list)

Given a list of tuples or :class:FrameSummary objects 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(exc, /[, value], *, show_group=False)

Format the exception part of a traceback using an exception value such as given by :data:sys.last_value. The return value is a list of strings, each ending in a newline. The list contains the exception's message, which is normally a single string; however, for :exc:SyntaxError exceptions, it contains several lines that (when printed) display detailed information about where the syntax error occurred. Following the message, the list contains the exception's :attr:notes <BaseException.__notes__>.

Since Python 3.10, instead of passing value, an exception object can be passed as the first argument. If value is provided, the first argument is ignored in order to provide backwards compatibility.

When show_group is True, and the exception is an instance of :exc:BaseExceptionGroup, the nested exceptions are included as well, recursively, with indentation relative to their nesting depth.

.. versionchanged:: 3.10 The etype parameter has been renamed to exc and is now positional-only.

.. versionchanged:: 3.11 The returned list now includes any :attr:notes <BaseException.__notes__> attached to the exception.

.. versionchanged:: 3.13 show_group parameter was added.

.. function:: format_exception(exc, /[, 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.

.. versionchanged:: 3.5 The etype argument is ignored and inferred from the type of value.

.. versionchanged:: 3.10 This function's behavior and signature were modified to match :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 :ref:traceback <traceback-objects> tb by calling the :meth:~frame.clear method of each :ref:frame object <frame-objects>.

.. versionadded:: 3.4

.. function:: walk_stack(f)

Walk a stack following :attr:f.f_back <frame.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

.. versionchanged:: 3.14 This function previously returned a generator that would walk the stack when first iterated over. The generator returned now is the state of the stack when walk_stack is called.

.. function:: walk_tb(tb)

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

.. versionadded:: 3.5

:class:!TracebackException Objects

.. versionadded:: 3.5

:class:!TracebackException objects are created from actual exceptions to capture data for later printing. They offer a more lightweight method of storing this information by avoiding holding references to :ref:traceback<traceback-objects> and :ref:frame<frame-objects> objects. In addition, they expose more options to configure the output compared to the module-level functions described above.

.. class:: TracebackException(exc_type, exc_value, exc_traceback, *, limit=None, lookup_lines=True, capture_locals=False, compact=False, max_group_width=15, max_group_depth=10)

Capture an exception for later rendering. The meaning of limit, lookup_lines and capture_locals are as for the :class:StackSummary class.

If compact is true, only data that is required by :class:!TracebackException's :meth:format method is saved in the class attributes. In particular, the :attr:__context__ field is calculated only if :attr:__cause__ is None and :attr:__suppress_context__ is false.

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

max_group_width and max_group_depth control the formatting of exception groups (see :exc:BaseExceptionGroup). The depth refers to the nesting level of the group, and the width refers to the size of a single exception group's exceptions array. The formatted output is truncated when either limit is exceeded.

.. versionchanged:: 3.10 Added the compact parameter.

.. versionchanged:: 3.11 Added the max_group_width and max_group_depth parameters.

.. attribute:: cause

  A :class:`!TracebackException` of the original
  :attr:`~BaseException.__cause__`.

.. attribute:: context

  A :class:`!TracebackException` of the original
  :attr:`~BaseException.__context__`.

.. attribute:: exceptions

  If ``self`` represents an :exc:`ExceptionGroup`, this field holds a list of
  :class:`!TracebackException` instances representing the nested exceptions.
  Otherwise it is ``None``.

  .. versionadded:: 3.11

.. attribute:: suppress_context

  The :attr:`~BaseException.__suppress_context__` value from the original
  exception.

.. attribute:: notes

  The :attr:`~BaseException.__notes__` value from the original exception,
  or ``None``
  if the exception does not have any notes. If it is not ``None``
  is it formatted in the traceback after the exception string.

  .. versionadded:: 3.11

.. attribute:: stack

  A :class:`StackSummary` representing the traceback.

.. attribute:: exc_type

  The class of the original traceback.

  .. deprecated:: 3.13

.. attribute:: exc_type_str

  String display of the class of the original exception.

  .. versionadded:: 3.13

.. 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:: end_lineno

  For syntax errors - the end line number where the error occurred.
  Can be ``None`` if not present.

  .. versionadded:: 3.10

.. 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:: end_offset

  For syntax errors - the end offset into the text where the error occurred.
  Can be ``None`` if not present.

  .. versionadded:: 3.10

.. 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:: print(*, file=None, chain=True)

  Print to *file* (default ``sys.stderr``) the exception information returned by
  :meth:`format`.

  .. versionadded:: 3.11

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

  Format the exception.

  If *chain* is not ``True``, :attr:`__cause__` and :attr:`__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.

.. method:: format_exception_only(*, show_group=False)

  Format the exception part of the traceback.

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

  When *show_group* is ``False``, the generator emits the exception's
  message followed by its notes (if it has any). The exception message
  is normally a single string; however, for :exc:`SyntaxError` exceptions,
  it consists of several lines that (when printed) display detailed
  information about where the syntax error occurred.

  When *show_group* is ``True``, and the exception is an instance of
  :exc:`BaseExceptionGroup`, the nested exceptions are included as
  well, recursively, with indentation relative to their nesting depth.

  .. versionchanged:: 3.11
     The exception's :attr:`notes <BaseException.__notes__>` are now
     included in the output.

  .. versionchanged:: 3.13
     Added the *show_group* parameter.

: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.

  .. versionchanged:: 3.12
     Exceptions raised from :func:`repr` on a local variable (when
     *capture_locals* is ``True``) are no longer propagated to the caller.

.. classmethod:: from_list(a_list)

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

.. method:: format()

  Returns a list of strings ready for printing.  Each string in the
  resulting list corresponds to a single :ref:`frame <frame-objects>` from
  the stack.
  Each string ends in a newline; the strings may contain internal
  newlines as well, for those items with source text lines.

  For long sequences of the same frame and line, the first few
  repetitions are shown, followed by a summary line stating the exact
  number of further repetitions.

  .. versionchanged:: 3.6
     Long sequences of repeated frames are now abbreviated.

.. method:: format_frame_summary(frame_summary)

  Returns a string for printing one of the :ref:`frames <frame-objects>`
  involved in the stack.
  This method is called for each :class:`FrameSummary` object to be
  printed by :meth:`StackSummary.format`. If it returns ``None``, the
  frame is omitted from the output.

  .. versionadded:: 3.11

:class:!FrameSummary Objects

.. versionadded:: 3.5

A :class:!FrameSummary object represents a single :ref:frame <frame-objects> in a :ref:traceback <traceback-objects>.

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

Represents a single :ref:frame <frame-objects> in the :ref:traceback <traceback-objects> or stack that is being formatted or printed. It may optionally have a stringified version of the frame's 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 :class:tuple). :attr:~FrameSummary.line may be directly provided, and will prevent line lookups happening at all. locals is an optional local variable mapping, and if supplied the variable representations are stored in the summary for later display.

:class:!FrameSummary instances have the following attributes:

.. attribute:: FrameSummary.filename

  The filename of the source code for this frame. Equivalent to accessing
  :attr:`f.f_code.co_filename <codeobject.co_filename>` on a
  :ref:`frame object <frame-objects>` *f*.

.. attribute:: FrameSummary.lineno

  The line number of the source code for this frame.

.. attribute:: FrameSummary.name

  Equivalent to accessing :attr:`f.f_code.co_name <codeobject.co_name>` on
  a :ref:`frame object <frame-objects>` *f*.

.. attribute:: FrameSummary.line

  A string representing the source code for this frame, with leading and
  trailing whitespace stripped.
  If the source is not available, it is ``None``.

.. attribute:: FrameSummary.end_lineno

  The last line number of the source code for this frame.
  By default, it is set to ``lineno`` and indexation starts from 1.

  .. versionchanged:: 3.13
     The default value changed from ``None`` to ``lineno``.

.. attribute:: FrameSummary.colno

  The column number of the source code for this frame.
  By default, it is ``None`` and indexation starts from 0.

.. attribute:: FrameSummary.end_colno

  The last column number of the source code for this frame.
  By default, it is ``None`` and indexation starts from 0.

.. _traceback-example:

Examples of Using the Module-Level Functions

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_life()

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

try: lumberjack() except IndexError as exc: print("*** print_tb:") traceback.print_tb(exc.traceback, limit=1, file=sys.stdout) print("*** print_exception:") traceback.print_exception(exc, limit=2, file=sys.stdout) print("*** print_exc:") traceback.print_exc(limit=2, file=sys.stdout) 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))) 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_life() ~~~~~~~~~~~~~~~~~~~^^ 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_life() ~~~~~~~~~~~~~~~~~~~^^ 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 default[0]>", line 10, in <module>\n lumberjack()\n ~~~~~~~~~~^^\n', ' File "<doctest default[0]>", line 4, in lumberjack\n bright_side_of_life()\n ~~~~~~~~~~~~~~~~~~~^^\n', ' File "<doctest default[0]>", line 7, in bright_side_of_life\n return tuple()[0]\n ~~~~~~~^^^\n', 'IndexError: tuple index out of range\n'] *** extract_tb: [<FrameSummary file <doctest...>, line 10 in <module>>, <FrameSummary file <doctest...>, line 4 in lumberjack>, <FrameSummary file <doctest...>, line 7 in bright_side_of_life>] *** format_tb: [' File "<doctest default[0]>", line 10, in <module>\n lumberjack()\n ~~~~~~~~~~^^\n', ' File "<doctest default[0]>", line 4, in lumberjack\n bright_side_of_life()\n ~~~~~~~~~~~~~~~~~~~^^\n', ' File "<doctest default[0]>", line 7, in bright_side_of_life\n return tuple()[0]\n ~~~~~~~^^^\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(an_error) ['IndexError: tuple index out of range\n']

Examples of Using :class:TracebackException

With the helper class, we have more options::

import sys from traceback import TracebackException

def lumberjack(): ... bright_side_of_life() ... def bright_side_of_life(): ... t = "bright", "side", "of", "life" ... return t[5] ... try: ... lumberjack() ... except IndexError as e: ... exc = e ... try: ... try: ... lumberjack() ... except: ... 1/0 ... except Exception as e: ... chained_exc = e ...

limit works as with the module-level functions

TracebackException.from_exception(exc, limit=-2).print() Traceback (most recent call last): File "<python-input-1>", line 6, in lumberjack bright_side_of_life() ~~~~~~~~~~~~~~~~~~~^^ File "<python-input-1>", line 10, in bright_side_of_life return t[5] ~^^^ IndexError: tuple index out of range

capture_locals adds local variables in frames

TracebackException.from_exception(exc, limit=-2, capture_locals=True).print() Traceback (most recent call last): File "<python-input-1>", line 6, in lumberjack bright_side_of_life() ~~~~~~~~~~~~~~~~~~~^^ File "<python-input-1>", line 10, in bright_side_of_life return t[5] ~^^^ t = ("bright", "side", "of", "life") IndexError: tuple index out of range

The chain kwarg to print() controls whether chained

exceptions are displayed

TracebackException.from_exception(chained_exc).print() Traceback (most recent call last): File "<python-input-19>", line 4, in <module> lumberjack() ~~~~~~~~~~^^ File "<python-input-8>", line 7, in lumberjack bright_side_of_life() ~~~~~~~~~~~~~~~~~~~^^ File "<python-input-8>", line 11, in bright_side_of_life return t[5] ~^^^ IndexError: tuple index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last): File "<python-input-19>", line 6, in <module> 1/0 ^ ZeroDivisionError: division by zero

TracebackException.from_exception(chained_exc).print(chain=False) Traceback (most recent call last): File "<python-input-19>", line 6, in <module> 1/0 ^ ZeroDivisionError: division by zero