traceback — Print or retrieve a stack traceback — Python 3.13.1 documentation
Description: Source code: 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 di...

# `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 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.

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

See also

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.

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.

- `TracebackException` class and its helper classes `StackSummary` and `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.

## Module-Level Functions

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.

  Note

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

  Changed in 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.

  Changed in version 3.5: The etype argument is ignored and inferred from the type of value.

  Changed in 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()`.

  Changed in 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.

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

  Changed in version 3.11: The returned list now includes any `notes` attached to the exception.

  Changed in 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()`.

  Changed in version 3.5: The etype argument is ignored and inferred from the type of value.

  Changed in 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.

## `TracebackException` Objects

Added in version 3.5.

`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 traceback and frame objects In addition, they expose more options to configure the output compared to the module-level functions described above.

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. The meaning of 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.

  Changed in version 3.10: Added the compact parameter.

  Changed in 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.

    Deprecated since 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.

    Changed in version 3.11: The exception’s `notes` are now included in the output.

    Changed in 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.

    Changed in 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.

    Changed in 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`.

## 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 `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 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:

    *** 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(an_error)
    ['IndexError: tuple index out of range\n']

## Examples of Using `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