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# 28.9. `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`(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]])
  Print exception information and up to limit stack trace entries from traceback to file. This differs from `print_tb()` in the following ways: (1) if traceback is not `None`, it prints a header `Traceback (most recent call last):`; (2) it prints the exception type and value after the stack trace; (3) 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.

`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`(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])
  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.

## 28.9.1. 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:
        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']

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