mirror of https://github.com/python/cpython
650 lines
23 KiB
ReStructuredText
650 lines
23 KiB
ReStructuredText
:mod:`!traceback` --- Print or retrieve a stack traceback
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=========================================================
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.. module:: traceback
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:synopsis: Print or retrieve a stack traceback.
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**Source code:** :source:`Lib/traceback.py`
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--------------
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This module provides a standard interface to extract, format and print stack
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traces of Python programs. It exactly mimics the behavior of the Python
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interpreter when it prints a stack trace. This is useful when you want to print
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stack traces under program control, such as in a "wrapper" around the
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interpreter.
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.. index:: pair: object; traceback
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The module uses :ref:`traceback objects <traceback-objects>` --- these are
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objects of type :class:`types.TracebackType`,
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which are assigned to the :attr:`~BaseException.__traceback__` field of
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:class:`BaseException` instances.
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.. seealso::
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Module :mod:`faulthandler`
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Used to dump Python tracebacks explicitly, on a fault, after a timeout, or on a user signal.
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Module :mod:`pdb`
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Interactive source code debugger for Python programs.
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The module defines the following functions:
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.. function:: print_tb(tb, limit=None, file=None)
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Print up to *limit* stack trace entries from
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:ref:`traceback object <traceback-objects>` *tb* (starting
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from the caller's frame) if *limit* is positive. Otherwise, print the last
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``abs(limit)`` entries. If *limit* is omitted or ``None``, all entries are
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printed. If *file* is omitted or ``None``, the output goes to
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:data:`sys.stderr`; otherwise it should be an open
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:term:`file <file object>` or :term:`file-like object` to
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receive the output.
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.. versionchanged:: 3.5
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Added negative *limit* support.
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.. function:: print_exception(exc, /[, value, tb], limit=None, \
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file=None, chain=True)
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Print exception information and stack trace entries from
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:ref:`traceback object <traceback-objects>`
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*tb* to *file*. This differs from :func:`print_tb` in the following
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ways:
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* if *tb* is not ``None``, it prints a header ``Traceback (most recent
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call last):``
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* it prints the exception type and *value* after the stack trace
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.. index:: single: ^ (caret); marker
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* if *type(value)* is :exc:`SyntaxError` and *value* has the appropriate
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format, it prints the line where the syntax error occurred with a caret
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indicating the approximate position of the error.
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Since Python 3.10, instead of passing *value* and *tb*, an exception object
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can be passed as the first argument. If *value* and *tb* are provided, the
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first argument is ignored in order to provide backwards compatibility.
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The optional *limit* argument has the same meaning as for :func:`print_tb`.
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If *chain* is true (the default), then chained exceptions (the
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:attr:`~BaseException.__cause__` or :attr:`~BaseException.__context__`
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attributes of the exception) will be
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printed as well, like the interpreter itself does when printing an unhandled
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exception.
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.. versionchanged:: 3.5
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The *etype* argument is ignored and inferred from the type of *value*.
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.. versionchanged:: 3.10
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The *etype* parameter has been renamed to *exc* and is now
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positional-only.
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.. function:: print_exc(limit=None, file=None, chain=True)
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This is a shorthand for ``print_exception(sys.exception(), limit, file,
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chain)``.
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.. function:: print_last(limit=None, file=None, chain=True)
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This is a shorthand for ``print_exception(sys.last_exc, limit, file,
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chain)``. In general it will work only after an exception has reached
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an interactive prompt (see :data:`sys.last_exc`).
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.. function:: print_stack(f=None, limit=None, file=None)
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Print up to *limit* stack trace entries (starting from the invocation
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point) if *limit* is positive. Otherwise, print the last ``abs(limit)``
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entries. If *limit* is omitted or ``None``, all entries are printed.
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The optional *f* argument can be used to specify an alternate
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:ref:`stack frame <frame-objects>`
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to start. The optional *file* argument has the same meaning as for
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:func:`print_tb`.
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.. versionchanged:: 3.5
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Added negative *limit* support.
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.. function:: extract_tb(tb, limit=None)
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Return a :class:`StackSummary` object representing a list of "pre-processed"
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stack trace entries extracted from the
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:ref:`traceback object <traceback-objects>` *tb*. It is useful
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for alternate formatting of stack traces. The optional *limit* argument has
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the same meaning as for :func:`print_tb`. A "pre-processed" stack trace
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entry is a :class:`FrameSummary` object containing attributes
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:attr:`~FrameSummary.filename`, :attr:`~FrameSummary.lineno`,
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:attr:`~FrameSummary.name`, and :attr:`~FrameSummary.line` representing the
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information that is usually printed for a stack trace.
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.. function:: extract_stack(f=None, limit=None)
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Extract the raw traceback from the current
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:ref:`stack frame <frame-objects>`. The return value has
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the same format as for :func:`extract_tb`. The optional *f* and *limit*
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arguments have the same meaning as for :func:`print_stack`.
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.. function:: format_list(extracted_list)
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Given a list of tuples or :class:`FrameSummary` objects as returned by
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:func:`extract_tb` or :func:`extract_stack`, return a list of strings ready
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for printing. Each string in the resulting list corresponds to the item with
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the same index in the argument list. Each string ends in a newline; the
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strings may contain internal newlines as well, for those items whose source
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text line is not ``None``.
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.. function:: format_exception_only(exc, /[, value], *, show_group=False)
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Format the exception part of a traceback using an exception value such as
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given by :data:`sys.last_value`. The return value is a list of strings, each
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ending in a newline. The list contains the exception's message, which is
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normally a single string; however, for :exc:`SyntaxError` exceptions, it
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contains several lines that (when printed) display detailed information
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about where the syntax error occurred. Following the message, the list
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contains the exception's :attr:`notes <BaseException.__notes__>`.
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Since Python 3.10, instead of passing *value*, an exception object
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can be passed as the first argument. If *value* is provided, the first
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argument is ignored in order to provide backwards compatibility.
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When *show_group* is ``True``, and the exception is an instance of
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:exc:`BaseExceptionGroup`, the nested exceptions are included as
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well, recursively, with indentation relative to their nesting depth.
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.. versionchanged:: 3.10
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The *etype* parameter has been renamed to *exc* and is now
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positional-only.
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.. versionchanged:: 3.11
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The returned list now includes any
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:attr:`notes <BaseException.__notes__>` attached to the exception.
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.. versionchanged:: 3.13
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*show_group* parameter was added.
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.. function:: format_exception(exc, /[, value, tb], limit=None, chain=True)
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Format a stack trace and the exception information. The arguments have the
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same meaning as the corresponding arguments to :func:`print_exception`. The
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return value is a list of strings, each ending in a newline and some
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containing internal newlines. When these lines are concatenated and printed,
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exactly the same text is printed as does :func:`print_exception`.
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.. versionchanged:: 3.5
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The *etype* argument is ignored and inferred from the type of *value*.
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.. versionchanged:: 3.10
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This function's behavior and signature were modified to match
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:func:`print_exception`.
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.. function:: format_exc(limit=None, chain=True)
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This is like ``print_exc(limit)`` but returns a string instead of printing to
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a file.
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.. function:: format_tb(tb, limit=None)
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A shorthand for ``format_list(extract_tb(tb, limit))``.
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.. function:: format_stack(f=None, limit=None)
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A shorthand for ``format_list(extract_stack(f, limit))``.
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.. function:: clear_frames(tb)
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Clears the local variables of all the stack frames in a
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:ref:`traceback <traceback-objects>` *tb*
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by calling the :meth:`~frame.clear` method of each
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:ref:`frame object <frame-objects>`.
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.. versionadded:: 3.4
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.. function:: walk_stack(f)
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Walk a stack following :attr:`f.f_back <frame.f_back>` from the given frame,
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yielding the frame
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and line number for each frame. If *f* is ``None``, the current stack is
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used. This helper is used with :meth:`StackSummary.extract`.
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.. versionadded:: 3.5
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.. function:: walk_tb(tb)
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Walk a traceback following :attr:`~traceback.tb_next` yielding the frame and
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line number
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for each frame. This helper is used with :meth:`StackSummary.extract`.
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.. versionadded:: 3.5
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The module also defines the following classes:
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:class:`!TracebackException` Objects
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------------------------------------
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.. versionadded:: 3.5
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:class:`!TracebackException` objects are created from actual exceptions to
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capture data for later printing in a lightweight fashion.
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.. 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)
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Capture an exception for later rendering. *limit*, *lookup_lines* and
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*capture_locals* are as for the :class:`StackSummary` class.
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If *compact* is true, only data that is required by
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:class:`!TracebackException`'s :meth:`format` method
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is saved in the class attributes. In particular, the
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:attr:`__context__` field is calculated only if :attr:`__cause__` is
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``None`` and :attr:`__suppress_context__` is false.
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Note that when locals are captured, they are also shown in the traceback.
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*max_group_width* and *max_group_depth* control the formatting of exception
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groups (see :exc:`BaseExceptionGroup`). The depth refers to the nesting
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level of the group, and the width refers to the size of a single exception
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group's exceptions array. The formatted output is truncated when either
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limit is exceeded.
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.. versionchanged:: 3.10
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Added the *compact* parameter.
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.. versionchanged:: 3.11
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Added the *max_group_width* and *max_group_depth* parameters.
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.. attribute:: __cause__
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A :class:`!TracebackException` of the original
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:attr:`~BaseException.__cause__`.
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.. attribute:: __context__
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A :class:`!TracebackException` of the original
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:attr:`~BaseException.__context__`.
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.. attribute:: exceptions
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If ``self`` represents an :exc:`ExceptionGroup`, this field holds a list of
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:class:`!TracebackException` instances representing the nested exceptions.
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Otherwise it is ``None``.
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.. versionadded:: 3.11
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.. attribute:: __suppress_context__
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The :attr:`~BaseException.__suppress_context__` value from the original
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exception.
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.. attribute:: __notes__
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The :attr:`~BaseException.__notes__` value from the original exception,
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or ``None``
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if the exception does not have any notes. If it is not ``None``
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is it formatted in the traceback after the exception string.
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.. versionadded:: 3.11
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.. attribute:: stack
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A :class:`StackSummary` representing the traceback.
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.. attribute:: exc_type
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The class of the original traceback.
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.. deprecated:: 3.13
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.. attribute:: exc_type_str
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String display of the class of the original exception.
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.. versionadded:: 3.13
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.. attribute:: filename
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For syntax errors - the file name where the error occurred.
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.. attribute:: lineno
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For syntax errors - the line number where the error occurred.
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.. attribute:: end_lineno
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For syntax errors - the end line number where the error occurred.
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Can be ``None`` if not present.
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.. versionadded:: 3.10
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.. attribute:: text
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For syntax errors - the text where the error occurred.
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.. attribute:: offset
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For syntax errors - the offset into the text where the error occurred.
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.. attribute:: end_offset
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For syntax errors - the end offset into the text where the error occurred.
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Can be ``None`` if not present.
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.. versionadded:: 3.10
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.. attribute:: msg
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For syntax errors - the compiler error message.
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.. classmethod:: from_exception(exc, *, limit=None, lookup_lines=True, capture_locals=False)
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Capture an exception for later rendering. *limit*, *lookup_lines* and
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*capture_locals* are as for the :class:`StackSummary` class.
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Note that when locals are captured, they are also shown in the traceback.
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.. method:: print(*, file=None, chain=True)
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Print to *file* (default ``sys.stderr``) the exception information returned by
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:meth:`format`.
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.. versionadded:: 3.11
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.. method:: format(*, chain=True)
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Format the exception.
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If *chain* is not ``True``, :attr:`__cause__` and :attr:`__context__`
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will not be formatted.
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The return value is a generator of strings, each ending in a newline and
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some containing internal newlines. :func:`~traceback.print_exception`
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is a wrapper around this method which just prints the lines to a file.
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.. method:: format_exception_only(*, show_group=False)
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Format the exception part of the traceback.
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The return value is a generator of strings, each ending in a newline.
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When *show_group* is ``False``, the generator emits the exception's
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message followed by its notes (if it has any). The exception message
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is normally a single string; however, for :exc:`SyntaxError` exceptions,
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it consists of several lines that (when printed) display detailed
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information about where the syntax error occurred.
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When *show_group* is ``True``, and the exception is an instance of
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:exc:`BaseExceptionGroup`, the nested exceptions are included as
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well, recursively, with indentation relative to their nesting depth.
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.. versionchanged:: 3.11
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The exception's :attr:`notes <BaseException.__notes__>` are now
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included in the output.
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.. versionchanged:: 3.13
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Added the *show_group* parameter.
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:class:`!StackSummary` Objects
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------------------------------
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.. versionadded:: 3.5
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:class:`!StackSummary` objects represent a call stack ready for formatting.
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.. class:: StackSummary
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.. classmethod:: extract(frame_gen, *, limit=None, lookup_lines=True, capture_locals=False)
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Construct a :class:`!StackSummary` object from a frame generator (such as
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is returned by :func:`~traceback.walk_stack` or
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:func:`~traceback.walk_tb`).
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If *limit* is supplied, only this many frames are taken from *frame_gen*.
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If *lookup_lines* is ``False``, the returned :class:`FrameSummary`
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objects will not have read their lines in yet, making the cost of
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creating the :class:`!StackSummary` cheaper (which may be valuable if it
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may not actually get formatted). If *capture_locals* is ``True`` the
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local variables in each :class:`!FrameSummary` are captured as object
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representations.
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.. versionchanged:: 3.12
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Exceptions raised from :func:`repr` on a local variable (when
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*capture_locals* is ``True``) are no longer propagated to the caller.
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.. classmethod:: from_list(a_list)
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Construct a :class:`!StackSummary` object from a supplied list of
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:class:`FrameSummary` objects or old-style list of tuples. Each tuple
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should be a 4-tuple with *filename*, *lineno*, *name*, *line* as the
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elements.
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.. method:: format()
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Returns a list of strings ready for printing. Each string in the
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resulting list corresponds to a single :ref:`frame <frame-objects>` from
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the stack.
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Each string ends in a newline; the strings may contain internal
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newlines as well, for those items with source text lines.
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For long sequences of the same frame and line, the first few
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repetitions are shown, followed by a summary line stating the exact
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number of further repetitions.
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.. versionchanged:: 3.6
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Long sequences of repeated frames are now abbreviated.
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.. method:: format_frame_summary(frame_summary)
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Returns a string for printing one of the :ref:`frames <frame-objects>`
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involved in the stack.
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This method is called for each :class:`FrameSummary` object to be
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printed by :meth:`StackSummary.format`. If it returns ``None``, the
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frame is omitted from the output.
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.. versionadded:: 3.11
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:class:`!FrameSummary` Objects
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------------------------------
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.. versionadded:: 3.5
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A :class:`!FrameSummary` object represents a single :ref:`frame <frame-objects>`
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in a :ref:`traceback <traceback-objects>`.
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.. class:: FrameSummary(filename, lineno, name, lookup_line=True, locals=None, line=None)
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Represents a single :ref:`frame <frame-objects>` in the
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:ref:`traceback <traceback-objects>` or stack that is being formatted
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or printed. It may optionally have a stringified version of the frame's
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locals included in it. If *lookup_line* is ``False``, the source code is not
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looked up until the :class:`!FrameSummary` has the :attr:`~FrameSummary.line`
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attribute accessed (which also happens when casting it to a :class:`tuple`).
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:attr:`~FrameSummary.line` may be directly provided, and will prevent line
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lookups happening at all. *locals* is an optional local variable
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dictionary, and if supplied the variable representations are stored in the
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summary for later display.
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:class:`!FrameSummary` instances have the following attributes:
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.. attribute:: FrameSummary.filename
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The filename of the source code for this frame. Equivalent to accessing
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:attr:`f.f_code.co_filename <codeobject.co_filename>` on a
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:ref:`frame object <frame-objects>` *f*.
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.. attribute:: FrameSummary.lineno
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The line number of the source code for this frame.
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.. attribute:: FrameSummary.name
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Equivalent to accessing :attr:`f.f_code.co_name <codeobject.co_name>` on
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a :ref:`frame object <frame-objects>` *f*.
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.. attribute:: FrameSummary.line
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A string representing the source code for this frame, with leading and
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trailing whitespace stripped.
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If the source is not available, it is ``None``.
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.. _traceback-example:
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Traceback Examples
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------------------
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This simple example implements a basic read-eval-print loop, similar to (but
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less useful than) the standard Python interactive interpreter loop. For a more
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complete implementation of the interpreter loop, refer to the :mod:`code`
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module. ::
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import sys, traceback
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def run_user_code(envdir):
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source = input(">>> ")
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try:
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exec(source, envdir)
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except Exception:
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print("Exception in user code:")
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print("-"*60)
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traceback.print_exc(file=sys.stdout)
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print("-"*60)
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envdir = {}
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while True:
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run_user_code(envdir)
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The following example demonstrates the different ways to print and format the
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exception and traceback:
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.. testcode::
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import sys, traceback
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def lumberjack():
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bright_side_of_life()
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def bright_side_of_life():
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return tuple()[0]
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try:
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lumberjack()
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except IndexError:
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exc = sys.exception()
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print("*** print_tb:")
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traceback.print_tb(exc.__traceback__, limit=1, file=sys.stdout)
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print("*** print_exception:")
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traceback.print_exception(exc, limit=2, file=sys.stdout)
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print("*** print_exc:")
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traceback.print_exc(limit=2, file=sys.stdout)
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print("*** format_exc, first and last line:")
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formatted_lines = traceback.format_exc().splitlines()
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print(formatted_lines[0])
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print(formatted_lines[-1])
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print("*** format_exception:")
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print(repr(traceback.format_exception(exc)))
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print("*** extract_tb:")
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print(repr(traceback.extract_tb(exc.__traceback__)))
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print("*** format_tb:")
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print(repr(traceback.format_tb(exc.__traceback__)))
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print("*** tb_lineno:", exc.__traceback__.tb_lineno)
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The output for the example would look similar to this:
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.. testoutput::
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:options: +NORMALIZE_WHITESPACE
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*** print_tb:
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File "<doctest...>", line 10, in <module>
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lumberjack()
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~~~~~~~~~~^^
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*** print_exception:
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Traceback (most recent call last):
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File "<doctest...>", line 10, in <module>
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lumberjack()
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~~~~~~~~~~^^
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File "<doctest...>", line 4, in lumberjack
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bright_side_of_life()
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~~~~~~~~~~~~~~~~~~~^^
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IndexError: tuple index out of range
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*** print_exc:
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Traceback (most recent call last):
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File "<doctest...>", line 10, in <module>
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lumberjack()
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~~~~~~~~~~^^
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File "<doctest...>", line 4, in lumberjack
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bright_side_of_life()
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~~~~~~~~~~~~~~~~~~~^^
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IndexError: tuple index out of range
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*** format_exc, first and last line:
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Traceback (most recent call last):
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IndexError: tuple index out of range
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*** format_exception:
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['Traceback (most recent call last):\n',
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' File "<doctest default[0]>", line 10, in <module>\n lumberjack()\n ~~~~~~~~~~^^\n',
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' File "<doctest default[0]>", line 4, in lumberjack\n bright_side_of_life()\n ~~~~~~~~~~~~~~~~~~~^^\n',
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' File "<doctest default[0]>", line 7, in bright_side_of_life\n return tuple()[0]\n ~~~~~~~^^^\n',
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'IndexError: tuple index out of range\n']
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*** extract_tb:
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[<FrameSummary file <doctest...>, line 10 in <module>>,
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<FrameSummary file <doctest...>, line 4 in lumberjack>,
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<FrameSummary file <doctest...>, line 7 in bright_side_of_life>]
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*** format_tb:
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[' File "<doctest default[0]>", line 10, in <module>\n lumberjack()\n ~~~~~~~~~~^^\n',
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' File "<doctest default[0]>", line 4, in lumberjack\n bright_side_of_life()\n ~~~~~~~~~~~~~~~~~~~^^\n',
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' File "<doctest default[0]>", line 7, in bright_side_of_life\n return tuple()[0]\n ~~~~~~~^^^\n']
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*** tb_lineno: 10
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The following example shows the different ways to print and format the stack::
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>>> import traceback
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>>> def another_function():
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... lumberstack()
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...
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>>> def lumberstack():
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... traceback.print_stack()
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... print(repr(traceback.extract_stack()))
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... print(repr(traceback.format_stack()))
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...
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>>> another_function()
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File "<doctest>", line 10, in <module>
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another_function()
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File "<doctest>", line 3, in another_function
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lumberstack()
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File "<doctest>", line 6, in lumberstack
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traceback.print_stack()
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[('<doctest>', 10, '<module>', 'another_function()'),
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('<doctest>', 3, 'another_function', 'lumberstack()'),
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('<doctest>', 7, 'lumberstack', 'print(repr(traceback.extract_stack()))')]
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[' File "<doctest>", line 10, in <module>\n another_function()\n',
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' File "<doctest>", line 3, in another_function\n lumberstack()\n',
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' File "<doctest>", line 8, in lumberstack\n print(repr(traceback.format_stack()))\n']
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This last example demonstrates the final few formatting functions:
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.. doctest::
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:options: +NORMALIZE_WHITESPACE
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>>> import traceback
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>>> traceback.format_list([('spam.py', 3, '<module>', 'spam.eggs()'),
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... ('eggs.py', 42, 'eggs', 'return "bacon"')])
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[' File "spam.py", line 3, in <module>\n spam.eggs()\n',
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' File "eggs.py", line 42, in eggs\n return "bacon"\n']
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>>> an_error = IndexError('tuple index out of range')
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>>> traceback.format_exception_only(type(an_error), an_error)
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['IndexError: tuple index out of range\n']
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