# 26.7 `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:

`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.

`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.

`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.)

`format_exc`( | [limit])

  This is like `print_exc(limit)` but returns a string instead of printing to a file. New in version 2.4.

`print_last`( | [limit[, file]])

  This is a shorthand for `print_exception(sys.last_type, sys.last_value, sys.last_traceback, limit, file)`.

`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()`.

`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`.

`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()`.

`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`.

`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.

`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()`.

`format_tb`( | tb[, limit])

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

`format_stack`( | [f[, limit]])

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

`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.