| :mod:`traceback` --- Print or retrieve a stack traceback |
| ======================================================== |
| |
| .. module:: traceback |
| :synopsis: Print or retrieve a stack traceback. |
| |
| |
| This module provides a standard interface to extract, format and print stack |
| traces of Python programs. It exactly mimics the behavior of the Python |
| interpreter when it prints a stack trace. This is useful when you want to print |
| stack traces under program control, such as in a "wrapper" around the |
| interpreter. |
| |
| .. index:: object: traceback |
| |
| The module uses traceback objects --- this is the object type that is stored in |
| the :data:`sys.last_traceback` variable and returned as the third item from |
| :func:`sys.exc_info`. |
| |
| The module defines the following functions: |
| |
| |
| .. function:: print_tb(traceback, limit=None, file=None) |
| |
| Print up to *limit* stack trace entries from *traceback*. 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. |
| |
| |
| .. function:: print_exception(type, value, traceback, limit=None, file=None, chain=True) |
| |
| Print exception information and up to *limit* stack trace entries from |
| *traceback* to *file*. This differs from :func:`print_tb` in the following |
| ways: |
| |
| * if *traceback* 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* 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. |
| |
| If *chain* is true (the default), then chained exceptions (the |
| :attr:`__cause__` or :attr:`__context__` attributes of the exception) will be |
| printed as well, like the interpreter itself does when printing an unhandled |
| exception. |
| |
| |
| .. function:: print_exc(limit=None, file=None, chain=True) |
| |
| This is a shorthand for ``print_exception(*sys.exc_info())``. |
| |
| |
| .. function:: print_last(limit=None, file=None, chain=True) |
| |
| This is a shorthand for ``print_exception(sys.last_type, sys.last_value, |
| sys.last_traceback, limit, file)``. In general it will work only after |
| an exception has reached an interactive prompt (see :data:`sys.last_type`). |
| |
| |
| .. function:: print_stack(f=None, limit=None, file=None) |
| |
| This function prints a stack trace from its invocation point. The optional *f* |
| argument can be used to specify an alternate stack frame to start. The optional |
| *limit* and *file* arguments have the same meaning as for |
| :func:`print_exception`. |
| |
| |
| .. function:: extract_tb(traceback, limit=None) |
| |
| Return a list of up to *limit* "pre-processed" stack trace entries extracted |
| from the traceback object *traceback*. It is useful for alternate formatting of |
| stack traces. If *limit* is omitted or ``None``, all entries are extracted. A |
| "pre-processed" stack trace entry is a quadruple (*filename*, *line number*, |
| *function name*, *text*) representing the information that is usually printed |
| for a stack trace. The *text* is a string with leading and trailing whitespace |
| stripped; if the source is not available it is ``None``. |
| |
| |
| .. function:: extract_stack(f=None, limit=None) |
| |
| Extract the raw traceback from the current stack frame. The return value has |
| the same format as for :func:`extract_tb`. The optional *f* and *limit* |
| arguments have the same meaning as for :func:`print_stack`. |
| |
| |
| .. function:: format_list(list) |
| |
| Given a list of tuples as returned by :func:`extract_tb` or |
| :func:`extract_stack`, return a list of strings ready for printing. Each string |
| in the resulting list corresponds to the item with the same index in the |
| argument list. Each string ends in a newline; the strings may contain internal |
| newlines as well, for those items whose source text line is not ``None``. |
| |
| |
| .. function:: format_exception_only(type, value) |
| |
| Format the exception part of a traceback. The arguments are the exception type |
| and value such as given by ``sys.last_type`` and ``sys.last_value``. The return |
| value is a list of strings, each ending in a newline. Normally, the list |
| contains a single string; however, for :exc:`SyntaxError` exceptions, it |
| contains several lines that (when printed) display detailed information about |
| where the syntax error occurred. The message indicating which exception |
| occurred is the always last string in the list. |
| |
| |
| .. function:: format_exception(type, value, tb, limit=None, chain=True) |
| |
| Format a stack trace and the exception information. The arguments have the |
| same meaning as the corresponding arguments to :func:`print_exception`. The |
| return value is a list of strings, each ending in a newline and some containing |
| internal newlines. When these lines are concatenated and printed, exactly the |
| same text is printed as does :func:`print_exception`. |
| |
| |
| .. function:: format_exc(limit=None, chain=True) |
| |
| This is like ``print_exc(limit)`` but returns a string instead of printing to a |
| file. |
| |
| |
| .. function:: format_tb(tb, limit=None) |
| |
| A shorthand for ``format_list(extract_tb(tb, limit))``. |
| |
| |
| .. function:: format_stack(f=None, limit=None) |
| |
| A shorthand for ``format_list(extract_stack(f, limit))``. |
| |
| .. function:: clear_frames(tb) |
| |
| Clears the local variables of all the stack frames in a traceback *tb* |
| by calling the :meth:`clear` method of each frame object. |
| |
| .. versionadded:: 3.4 |
| |
| |
| .. _traceback-example: |
| |
| Traceback Examples |
| ------------------ |
| |
| This simple example implements a basic read-eval-print loop, similar to (but |
| less useful than) the standard Python interactive interpreter loop. For a more |
| complete implementation of the interpreter loop, refer to the :mod:`code` |
| module. :: |
| |
| import sys, traceback |
| |
| def run_user_code(envdir): |
| source = input(">>> ") |
| try: |
| exec(source, envdir) |
| except Exception: |
| print("Exception in user code:") |
| print("-"*60) |
| traceback.print_exc(file=sys.stdout) |
| print("-"*60) |
| |
| envdir = {} |
| while True: |
| run_user_code(envdir) |
| |
| |
| The following example demonstrates the different ways to print and format the |
| exception and traceback: |
| |
| .. testcode:: |
| |
| import sys, traceback |
| |
| def lumberjack(): |
| bright_side_of_death() |
| |
| def bright_side_of_death(): |
| return tuple()[0] |
| |
| try: |
| lumberjack() |
| except IndexError: |
| exc_type, exc_value, exc_traceback = sys.exc_info() |
| print("*** print_tb:") |
| traceback.print_tb(exc_traceback, limit=1, file=sys.stdout) |
| print("*** print_exception:") |
| traceback.print_exception(exc_type, exc_value, exc_traceback, |
| limit=2, file=sys.stdout) |
| print("*** print_exc:") |
| traceback.print_exc() |
| print("*** format_exc, first and last line:") |
| formatted_lines = traceback.format_exc().splitlines() |
| print(formatted_lines[0]) |
| print(formatted_lines[-1]) |
| print("*** format_exception:") |
| print(repr(traceback.format_exception(exc_type, exc_value, |
| exc_traceback))) |
| print("*** extract_tb:") |
| print(repr(traceback.extract_tb(exc_traceback))) |
| print("*** format_tb:") |
| print(repr(traceback.format_tb(exc_traceback))) |
| print("*** tb_lineno:", exc_traceback.tb_lineno) |
| |
| The output for the example would look similar to this: |
| |
| .. testoutput:: |
| :options: +NORMALIZE_WHITESPACE |
| |
| *** print_tb: |
| File "<doctest...>", line 10, in <module> |
| lumberjack() |
| *** print_exception: |
| Traceback (most recent call last): |
| File "<doctest...>", line 10, in <module> |
| lumberjack() |
| File "<doctest...>", line 4, in lumberjack |
| bright_side_of_death() |
| IndexError: tuple index out of range |
| *** print_exc: |
| Traceback (most recent call last): |
| File "<doctest...>", line 10, in <module> |
| lumberjack() |
| File "<doctest...>", line 4, in lumberjack |
| bright_side_of_death() |
| IndexError: tuple index out of range |
| *** format_exc, first and last line: |
| Traceback (most recent call last): |
| IndexError: tuple index out of range |
| *** format_exception: |
| ['Traceback (most recent call last):\n', |
| ' File "<doctest...>", line 10, in <module>\n lumberjack()\n', |
| ' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n', |
| ' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n', |
| 'IndexError: tuple index out of range\n'] |
| *** extract_tb: |
| [('<doctest...>', 10, '<module>', 'lumberjack()'), |
| ('<doctest...>', 4, 'lumberjack', 'bright_side_of_death()'), |
| ('<doctest...>', 7, 'bright_side_of_death', 'return tuple()[0]')] |
| *** format_tb: |
| [' File "<doctest...>", line 10, in <module>\n lumberjack()\n', |
| ' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n', |
| ' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n'] |
| *** tb_lineno: 10 |
| |
| |
| The following example shows the different ways to print and format the stack:: |
| |
| >>> import traceback |
| >>> def another_function(): |
| ... lumberstack() |
| ... |
| >>> def lumberstack(): |
| ... traceback.print_stack() |
| ... print(repr(traceback.extract_stack())) |
| ... print(repr(traceback.format_stack())) |
| ... |
| >>> another_function() |
| File "<doctest>", line 10, in <module> |
| another_function() |
| File "<doctest>", line 3, in another_function |
| lumberstack() |
| File "<doctest>", line 6, in lumberstack |
| traceback.print_stack() |
| [('<doctest>', 10, '<module>', 'another_function()'), |
| ('<doctest>', 3, 'another_function', 'lumberstack()'), |
| ('<doctest>', 7, 'lumberstack', 'print(repr(traceback.extract_stack()))')] |
| [' File "<doctest>", line 10, in <module>\n another_function()\n', |
| ' File "<doctest>", line 3, in another_function\n lumberstack()\n', |
| ' File "<doctest>", line 8, in lumberstack\n print(repr(traceback.format_stack()))\n'] |
| |
| |
| This last example demonstrates the final few formatting functions: |
| |
| .. doctest:: |
| :options: +NORMALIZE_WHITESPACE |
| |
| >>> import traceback |
| >>> traceback.format_list([('spam.py', 3, '<module>', 'spam.eggs()'), |
| ... ('eggs.py', 42, 'eggs', 'return "bacon"')]) |
| [' File "spam.py", line 3, in <module>\n spam.eggs()\n', |
| ' File "eggs.py", line 42, in eggs\n return "bacon"\n'] |
| >>> an_error = IndexError('tuple index out of range') |
| >>> traceback.format_exception_only(type(an_error), an_error) |
| ['IndexError: tuple index out of range\n'] |