traceback --- Print or retrieve a stack traceback

Source code: 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.

The module uses traceback objects --- these are objects of type types.TracebackType, which are assigned to the __traceback__ field of BaseException instances.

Voir aussi

Module faulthandler

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

Module pdb

Interactive source code debugger for Python programs.

Le module définit les fonctions suivantes :

traceback.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.

Modifié dans la version 3.5: Added negative limit support.

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

Print exception information and stack trace entries from traceback object tb to file. This differs from 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

  • if type(value) is 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 print_tb(). If chain is true (the default), then chained exceptions (the __cause__ or __context__ attributes of the exception) will be printed as well, like the interpreter itself does when printing an unhandled exception.

Modifié dans la version 3.5: The etype argument is ignored and inferred from the type of value.

Modifié dans la version 3.10: The etype parameter has been renamed to exc and is now positional-only.

traceback.print_exc(limit=None, file=None, chain=True)

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

traceback.print_last(limit=None, file=None, chain=True)

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

traceback.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 print_tb().

Modifié dans la version 3.5: Added negative limit support.

traceback.extract_tb(tb, limit=None)

Return a StackSummary object representing 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 print_tb(). A "pre-processed" stack trace entry is a FrameSummary object containing attributes filename, lineno, name, and line representing the information that is usually printed for a stack trace.

traceback.extract_stack(f=None, limit=None)

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

traceback.format_list(extracted_list)

Given a list of tuples or FrameSummary objects as returned by extract_tb() or 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.

traceback.format_exception_only(exc, /, [value, ]*, show_group=False)

Format the exception part of a traceback using an exception value such as given by 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 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 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 BaseExceptionGroup, the nested exceptions are included as well, recursively, with indentation relative to their nesting depth.

Modifié dans la version 3.10: The etype parameter has been renamed to exc and is now positional-only.

Modifié dans la version 3.11: The returned list now includes any notes attached to the exception.

Modifié dans la version 3.13: show_group parameter was added.

traceback.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 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 print_exception().

Modifié dans la version 3.5: The etype argument is ignored and inferred from the type of value.

Modifié dans la version 3.10: This function's behavior and signature were modified to match print_exception().

traceback.format_exc(limit=None, chain=True)

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

traceback.format_tb(tb, limit=None)

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

traceback.format_stack(f=None, limit=None)

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

traceback.clear_frames(tb)

Clears the local variables of all the stack frames in a traceback tb by calling the clear() method of each frame object.

Added in version 3.4.

traceback.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 StackSummary.extract().

Added in version 3.5.

traceback.walk_tb(tb)

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

Added in version 3.5.

The module also defines the following classes:

TracebackException Objects

Added in version 3.5.

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

class traceback.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. limit, lookup_lines and capture_locals are as for the StackSummary class.

If compact is true, only data that is required by TracebackException's format() method is saved in the class attributes. In particular, the __context__ field is calculated only if __cause__ is None and __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 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.

Modifié dans la version 3.10: Added the compact parameter.

Modifié dans la version 3.11: Added the max_group_width and max_group_depth parameters.

__cause__

A TracebackException of the original __cause__.

__context__

A TracebackException of the original __context__.

exceptions

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

Added in version 3.11.

__suppress_context__

The __suppress_context__ value from the original exception.

__notes__

The __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.

Added in version 3.11.

stack

A StackSummary representing the traceback.

exc_type

The class of the original traceback.

Obsolète depuis la version 3.13.

exc_type_str

String display of the class of the original exception.

Added in version 3.13.

filename

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

lineno

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

end_lineno

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

Added in version 3.10.

text

For syntax errors - the text where the error occurred.

offset

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

end_offset

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

Added in version 3.10.

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 StackSummary class.

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

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

Print to file (default sys.stderr) the exception information returned by format().

Added in version 3.11.

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. print_exception() is a wrapper around this method which just prints the lines to a file.

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 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 BaseExceptionGroup, the nested exceptions are included as well, recursively, with indentation relative to their nesting depth.

Modifié dans la version 3.11: The exception's notes are now included in the output.

Modifié dans la version 3.13: Added the show_group parameter.

StackSummary Objects

Added in version 3.5.

StackSummary objects represent a call stack ready for formatting.

class traceback.StackSummary
classmethod extract(frame_gen, *, limit=None, lookup_lines=True, capture_locals=False)

Construct a StackSummary object from a frame generator (such as is returned by walk_stack() or walk_tb()).

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

Modifié dans la version 3.12: Exceptions raised from repr() on a local variable (when capture_locals is True) are no longer propagated to the caller.

classmethod from_list(a_list)

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

format()

Returns a list of strings ready for printing. Each string in the resulting list corresponds to a single frame 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.

Modifié dans la version 3.6: Long sequences of repeated frames are now abbreviated.

format_frame_summary(frame_summary)

Returns a string for printing one of the frames involved in the stack. This method is called for each FrameSummary object to be printed by StackSummary.format(). If it returns None, the frame is omitted from the output.

Added in version 3.11.

FrameSummary Objects

Added in version 3.5.

A FrameSummary object represents a single frame in a traceback.

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

Represents a single frame in the traceback 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 FrameSummary has the line attribute accessed (which also happens when casting it to a tuple). 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.

FrameSummary instances have the following attributes:

filename

The filename of the source code for this frame. Equivalent to accessing f.f_code.co_filename on a frame object f.

lineno

The line number of the source code for this frame.

name

Equivalent to accessing f.f_code.co_name on a frame object f.

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.

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 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:

import sys, traceback

def lumberjack():
    bright_side_of_life()

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

try:
    lumberjack()
except IndexError:
    exc = sys.exception()
    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:

*** 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:

>>> 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']