28.10. "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 variables "sys.exc_traceback" (deprecated) and
"sys.last_traceback" and returned as the third item from
"sys.exc_info()".

The module defines the following functions:

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

   Print up to *limit* stack trace entries from the traceback object
   *tb*. 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(etype, value, tb[, limit[, file]])

   Print exception information and up to *limit* stack trace entries
   from the traceback *tb* to *file*. This differs from "print_tb()"
   in the following ways: (1) if *tb* is not "None", it prints a
   header "Traceback (most recent call last):"; (2) it prints the
   exception *etype* and *value* after the stack trace; (3) if *etype*
   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.

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

   This is a shorthand for "print_exception(sys.exc_type,
   sys.exc_value, sys.exc_traceback, limit, file)".  (In fact, it uses
   "sys.exc_info()" to retrieve the same information in a thread-safe
   way instead of using the deprecated variables.)

traceback.format_exc([limit])

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

   New in version 2.4.

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

   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(tb[, limit])

   Return a list of up to *limit* “pre-processed” stack trace entries
   extracted from the traceback object *tb*.  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 4-tuple (*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(extracted_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(etype, value)

   Format the exception part of a traceback.  The arguments are the
   exception type, *etype* 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(etype, value, tb[, limit])

   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_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.tb_lineno(tb)

   This function returns the current line number set in the traceback
   object. This function was necessary because in versions of Python
   prior to 2.3 when the "-O" flag was passed to Python the
   "tb.tb_lineno" was not updated correctly.  This function has no use
   in versions past 2.3.


28.10.1. 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 = raw_input(">>> ")
       try:
           exec source in envdir
       except:
           print "Exception in user code:"
           print '-'*60
           traceback.print_exc(file=sys.stdout)
           print '-'*60

   envdir = {}
   while 1:
       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 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:

   *** 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']
