
``traceback`` --- 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.

The module uses traceback objects --- this is the object type that is
stored in the ``sys.last_traceback`` variable and returned as the
third item from ``sys.exc_info()``.

The module defines the following functions:

traceback.print_tb(traceback[, limit[, file]])

   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.

traceback.print_exception(type, value, traceback[, limit[, file[, chain]]])

   Print exception information and up to *limit* stack trace entries
   from *traceback* to *file*. This differs from ``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 ``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
   ``__cause__`` or ``__context__`` attributes of the exception) will
   be printed as well, like the interpreter itself does when printing
   an unhandled exception.

traceback.print_exc([limit[, file[, chain]]])

   This is a shorthand for ``print_exception(*sys.exc_info())``.

traceback.print_last([limit[, file[, chain]]])

   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 ``sys.last_type``).

traceback.print_stack([f[, limit[, file]]])

   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 ``print_exception()``.

traceback.extract_tb(traceback[, limit])

   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``.

traceback.extract_stack([f[, limit]])

   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(list)

   Given a list of tuples 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(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 ``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.

traceback.format_exception(type, value, tb[, limit[, chain]])

   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()``.

traceback.format_exc([limit[, chain]])

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

traceback.format_tb(tb[, limit])

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

traceback.format_stack([f[, limit]])

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


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:
           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_death()

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

   try:
       lumberjack()
   except:
       exceptionType, exceptionValue, exceptionTraceback = sys.exc_info()
       print("*** print_tb:")
       traceback.print_tb(exceptionTraceback, limit=1, file=sys.stdout)
       print("*** print_exception:")
       traceback.print_exception(exceptionType, exceptionValue, exceptionTraceback,
                                 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(exceptionType, exceptionValue,
                                             exceptionTraceback)))
       print("*** extract_tb:")
       print(repr(traceback.extract_tb(exceptionTraceback)))
       print("*** format_tb:")
       print(repr(traceback.format_tb(exceptionTraceback)))
       print("*** tb_lineno:", traceback.tb_lineno(exceptionTraceback))

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_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:

   >>> 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']
