====================================================================
traceback -- Extract, format, and print exceptions and stack traces.
====================================================================
.. module:: traceback
:synopsis: Extract, format, and print exceptions and stack traces.
:Purpose: Extract, format, and print exceptions and stack traces.
:Python Version: 1.4 and later, with modifications over time
The :mod:`traceback` module works with the call stack to produce error
messages. A traceback is a stack trace from the point of an exception
handler down the call chain to the point where the exception was
raised. You can also work with the current call stack up from the
point of a call (and without the context of an error), which is useful
for finding out the paths being followed into a function.
The functions in :mod:`traceback` fall into several common categories.
There are functions for extracting raw tracebacks from the current
runtime environment (either an exception handler for a traceback, or
the regular stack). The extracted stack trace is a sequence of tuples
containing the filename, line number, function name, and text of the
source line.
Once extracted, the stack trace can be formatted using functions like
:func:`format_exception()`, :func:`format_stack()`, etc. The format
functions return a list of strings with messages formatted to be
printed. There are shorthand functions for printing the formatted
values, as well.
Although the functions in :mod:`traceback` mimic the behavior of the
interactive interpreter by default, they also are useful for handling
exceptions in situations where dumping the full stack trace to stderr
is not desirable. For example, a web application may need to format
the traceback so it looks good in HTML. An IDE may convert the
elements of the stack trace into a clickable list that lets the user
browse the source.
Supporting Functions
====================
The examples below use the module traceback_example.py (provided in
the source package for PyMOTW). The contents are:
.. include:: traceback_example.py
:literal:
:start-after: #end_pymotw_header
Working With Exceptions
=======================
The simplest way to handle exception reporting is with
:func:`print_exc()`. It uses :func:`sys.exc_info()` to obtain the
exception information for the current thread, formats the results, and
prints the text to a file handle (``sys.stderr``, by default).
.. include:: traceback_print_exc.py
:literal:
:start-after: #end_pymotw_header
In this example, the file handle for ``sys.stdout`` is substituted so
the informational and traceback messages are mingled correctly:
.. {{{cog
.. cog.out(run_script(cog.inFile, 'traceback_print_exc.py'))
.. }}}
::
$ python traceback_print_exc.py
print_exc() with no exception:
None
print_exc():
Traceback (most recent call last):
File "traceback_print_exc.py", line 20, in <module>
produce_exception()
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 16, in produce_exception
produce_exception(recursion_level-1)
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 16, in produce_exception
produce_exception(recursion_level-1)
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 18, in produce_exception
raise RuntimeError()
RuntimeError
print_exc(1):
Traceback (most recent call last):
File "traceback_print_exc.py", line 20, in <module>
produce_exception()
RuntimeError
.. {{{end}}}
``print_exc()`` is just a shortcut for :func:`print_exception()`,
which requires explicit arguments:
.. include:: traceback_print_exception.py
:literal:
:start-after: #end_pymotw_header
.. {{{cog
.. cog.out(run_script(cog.inFile, 'traceback_print_exception.py'))
.. }}}
::
$ python traceback_print_exception.py
Traceback (most recent call last):
File "traceback_print_exception.py", line 16, in <module>
produce_exception()
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 16, in produce_exception
produce_exception(recursion_level-1)
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 16, in produce_exception
produce_exception(recursion_level-1)
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 18, in produce_exception
raise RuntimeError()
RuntimeError
print_exception():
.. {{{end}}}
And :func:`print_exception()` uses :func:`format_exception()`:
.. include:: traceback_format_exception.py
:literal:
:start-after: #end_pymotw_header
.. {{{cog
.. cog.out(run_script(cog.inFile, 'traceback_format_exception.py'))
.. }}}
::
$ python traceback_format_exception.py
format_exception():
['Traceback (most recent call last):\n',
' File "traceback_format_exception.py", line 17, in <module>\n produce_exception()\n',
' File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 16, in produce_exception\n produce_exception(recursion_level-1)\n',
' File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 16, in produce_exception\n produce_exception(recursion_level-1)\n',
' File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 18, in produce_exception\n raise RuntimeError()\n',
'RuntimeError\n']
.. {{{end}}}
Working With the Stack
======================
There are a similar set of functions for performing the same operations with
the current call stack instead of a traceback.
print_stack()
-------------
.. include:: traceback_print_stack.py
:literal:
:start-after: #end_pymotw_header
.. {{{cog
.. cog.out(run_script(cog.inFile, 'traceback_print_stack.py'))
.. }}}
::
$ python traceback_print_stack.py
Calling f() directly:
File "traceback_print_stack.py", line 19, in <module>
f()
File "traceback_print_stack.py", line 16, in f
traceback.print_stack(file=sys.stdout)
Calling f() from 3 levels deep:
File "traceback_print_stack.py", line 23, in <module>
call_function(f)
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 22, in call_function
return call_function(f, recursion_level-1)
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 22, in call_function
return call_function(f, recursion_level-1)
File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 24, in call_function
return f()
File "traceback_print_stack.py", line 16, in f
traceback.print_stack(file=sys.stdout)
.. {{{end}}}
format_stack()
--------------
.. include:: traceback_format_stack.py
:literal:
:start-after: #end_pymotw_header
.. {{{cog
.. cog.out(run_script(cog.inFile, 'traceback_format_stack.py'))
.. }}}
::
$ python traceback_format_stack.py
[' File "traceback_format_stack.py", line 19, in <module>\n formatted_stack = call_function(f)\n',
' File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 22, in call_function\n return call_function(f, recursion_level-1)\n',
' File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 22, in call_function\n return call_function(f, recursion_level-1)\n',
' File "/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py", line 24, in call_function\n return f()\n',
' File "traceback_format_stack.py", line 17, in f\n return traceback.format_stack()\n']
.. {{{end}}}
extract_stack()
---------------
.. include:: traceback_extract_stack.py
:literal:
:start-after: #end_pymotw_header
.. {{{cog
.. cog.out(run_script(cog.inFile, 'traceback_extract_stack.py'))
.. }}}
::
$ python traceback_extract_stack.py
[('traceback_extract_stack.py', 19, '<module>', 'stack = call_function(f)'),
('/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py',
22,
'call_function',
'return call_function(f, recursion_level-1)'),
('/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py',
22,
'call_function',
'return call_function(f, recursion_level-1)'),
('/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/traceback/traceback_example.py',
24,
'call_function',
'return f()'),
('traceback_extract_stack.py', 17, 'f', 'return traceback.extract_stack()')]
.. {{{end}}}
.. seealso::
`traceback <http://docs.python.org/lib/module-traceback.html>`_
Standard library documentation for this module.
:mod:`sys`
The sys module includes singletons that hold the current exception.
:mod:`inspect`
The inspect module includes other functions for probing the frames on the stack.
:mod:`cgitb`
Another module for formatting tracebacks nicely.