29.9. "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 — 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(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.

   Changed in version 3.5: Added negative *limit* support.

traceback.print_exception(etype, 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 *etype* 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.

   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*.

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

   This is a shorthand for "print_exception(*sys.exc_info(), limit,
   file, chain)".

traceback.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, chain)".  In
   general it will work only after an exception has reached an
   interactive prompt (see "sys.last_type").

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 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 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=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 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 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=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*.

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.

   New 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()".

   New 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()".

   New in version 3.5.

The module also defines the following classes:


29.9.1. "TracebackException" Objects
====================================

New 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)

   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.

   __cause__

      A "TracebackException" of the original "__cause__".

   __context__

      A "TracebackException" of the original "__context__".

   __suppress_context__

      The "__suppress_context__" value from the original exception.

   stack

      A "StackSummary" representing the traceback.

   exc_type

      The class of the original traceback.

   filename

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

   lineno

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

   text

      For syntax errors - the text where the error occurred.

   offset

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

   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.

   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.

      The message indicating which exception occurred is always the
      last string in the output.

   format_exception_only()

      Format the exception part of the traceback.

      The return value is a generator of strings, each ending in a
      newline.

      Normally, the generator emits a single string; however, for
      "SyntaxError" exceptions, it emits several lines that (when
      printed) display detailed information about where the syntax
      error occurred.

      The message indicating which exception occurred is always the
      last string in the output.


29.9.2. "StackSummary" Objects
==============================

New 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.

   classmethod from_list(a_list)

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


29.9.3. "FrameSummary" Objects
==============================

New in version 3.5.

"FrameSummary" objects represent a single frame in a traceback.

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

   Represent a single frame in the traceback or stack that is being
   formatted or printed. It may optionally have a stringified version
   of the frames 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
   dictionary, and if supplied the variable representations are stored
   in the summary for later display.


29.9.4. 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_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:")
       # exc_type below is ignored on 3.5 and later
       traceback.print_exception(exc_type, exc_value, exc_traceback,
                                 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:")
       # exc_type below is ignored on 3.5 and later
       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:
   [<FrameSummary file <doctest...>, line 10 in <module>>,
    <FrameSummary file <doctest...>, line 4 in lumberjack>,
    <FrameSummary file <doctest...>, line 7 in bright_side_of_death>]
   *** 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']
