.. _trouble:

=========================
Things that cause trouble
=========================

:history: 20121231T085200, brand new docs.

Coverage works well, and I want it to properly measure any Python program, but
there are some situations it can't cope with.  This page details some known
problems, with possible courses of action, and links to coverage.py bug reports
with more information.

I would love to :ref:`hear from you <contact>` if you have information about
any of these problems, even just to explain to me why you want them to start
working properly.

If your problem isn't discussed here, you can of course search the `coverage.py
bug tracker`_ directly to see if there is some mention of it.

.. _coverage.py bug tracker: https://bitbucket.org/ned/coveragepy/issues?status=new&status=open


Things that don't work
----------------------

There are a number of popular modules, packages, and libraries that prevent
coverage.py from working properly:

* `execv`_, or one of its variants.  These end the current program and replace
  it with a new one.  This doesn't save the collected coverage data, so your
  program that calls execv will not be fully measured.  A patch for coverage.py
  is in `issue 43`_.

* `multiprocessing`_ launches processes to provide parallelism.  These
  processes don't get measured by coverage.py.  Some possible fixes are
  discussed or linked to in `issue 117`_.

* `gevent`_, which is based on `greenlet`_, and is similar to `eventlet`_. All
  of these manipulate the C stack, and therefore confuse coverage.py.
  `Issue 149`_ has some pointers to more information.

* `sys.settrace`_ is the Python feature that coverage.py uses to see what's
  happening in your program.  If another part of your program is using
  sys.settrace, then it will conflict with coverage.py, and it won't be
  measured properly.

.. _execv: http://docs.python.org/library/os#os.execl
.. _multiprocessing: http://docs.python.org/library/multiprocessing.html
.. _gevent: http://www.gevent.org/
.. _greenlet: http://greenlet.readthedocs.org/
.. _eventlet: http://eventlet.net/
.. _sys.settrace: docs.python.org/library/sys.html#sys.settrace
.. _issue 43: https://bitbucket.org/ned/coveragepy/issue/43/coverage-measurement-fails-on-code
.. _issue 117: https://bitbucket.org/ned/coveragepy/issue/117/enable-coverage-measurement-of-code-run-by
.. _issue 149: https://bitbucket.org/ned/coveragepy/issue/149/coverage-gevent-looks-broken


Things that require --timid
---------------------------

Some packages interfere with coverage measurement, but you might be able to
make it work by using the ``--timid`` command-line switch, or the ``[run]
timid=True`` configuration option.

* `DecoratorTools`_, or any package which uses it, notably `TurboGears`_.
  DecoratorTools fiddles with the trace function.  You  will need to use
  ``--timid``.

.. _DecoratorTools: http://pypi.python.org/pypi/DecoratorTools
.. _TurboGears: http://turbogears.org/


Really obscure things
---------------------

* Python 2.5 had a bug (`1569356`_) that could make your program behave
  differently when being measured with coverage.  This is diagnosed in
  `issue 51`_.

.. _issue 51: http://bitbucket.org/ned/coveragepy/issue/51/turbogears-15-test-failing-with-coverage
.. _1569356: http://bugs.python.org/issue1569356


Still having trouble?
---------------------

If your problem isn't mentioned here, and isn't already reported in the
`coverage.py bug tracker`_, please :ref:`get in touch with me <contact>`,
we'll figure out a solution.
