Some Issues and Questions
==================================

.. note::

    This FAQ is here only mostly for historic reasons.  Checkout
    `pytest Q&A at Stackoverflow <http://stackoverflow.com/search?q=pytest>`_
    for many questions and answers related to pytest and/or use
    :ref:`contact channels` to get help.

On naming, nosetests, licensing and magic
------------------------------------------------

How does pytest relate to nose and unittest?
+++++++++++++++++++++++++++++++++++++++++++++++++

``pytest`` and nose_ share basic philosophy when it comes
to running and writing Python tests.  In fact, you can run many tests
written for nose with ``pytest``.  nose_ was originally created
as a clone of ``pytest`` when ``pytest`` was in the ``0.8`` release
cycle.  Note that starting with pytest-2.0 support for running unittest
test suites is majorly improved.

how does pytest relate to twisted's trial?
++++++++++++++++++++++++++++++++++++++++++++++

Since some time ``pytest`` has builtin support for supporting tests
written using trial. It does not itself start a reactor, however,
and does not handle Deferreds returned from a test in pytest style.
If you are using trial's unittest.TestCase chances are that you can
just run your tests even if you return Deferreds.  In addition,
there also is a dedicated `pytest-twisted
<http://pypi.python.org/pypi/pytest-twisted>`_ plugin which allows you to
return deferreds from pytest-style tests, allowing the use of
:ref:`fixtures` and other features.

how does pytest work with Django?
++++++++++++++++++++++++++++++++++++++++++++++

In 2012, some work is going into the `pytest-django plugin <http://pypi.python.org/pypi/pytest-django>`_.  It substitutes the usage of Django's
``manage.py test`` and allows the use of all pytest features_ most of which
are not available from Django directly.

.. _features: features.html


What's this "magic" with pytest? (historic notes)
++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Around 2007 (version ``0.8``) some people thought that ``pytest``
was using too much "magic".  It had been part of the `pylib`_ which
contains a lot of unrelated python library code.  Around 2010 there
was a major cleanup refactoring, which removed unused or deprecated code
and resulted in the new ``pytest`` PyPI package which strictly contains
only test-related code.  This release also brought a complete pluginification
such that the core is around 300 lines of code and everything else is
implemented in plugins.  Thus ``pytest`` today is a small, universally runnable
and customizable testing framework for Python.   Note, however, that
``pytest`` uses metaprogramming techniques and reading its source is
thus likely not something for Python beginners.

A second "magic" issue was the assert statement debugging feature.
Nowadays, ``pytest`` explicitly rewrites assert statements in test modules
in order to provide more useful :ref:`assert feedback <assertfeedback>`.
This completely avoids previous issues of confusing assertion-reporting.
It also means, that you can use Python's ``-O`` optimization without losing
assertions in test modules.

``pytest`` contains a second, mostly obsolete, assert debugging technique
invoked via ``--assert=reinterpret``: When an ``assert`` statement fails, ``pytest`` re-interprets
the expression part to show intermediate values.  This technique suffers
from a caveat that the rewriting does not: If your expression has side
effects (better to avoid them anyway!) the intermediate values may not
be the same, confusing the reinterpreter and obfuscating the initial
error (this is also explained at the command line if it happens).

You can also turn off all assertion interaction using the
``--assert=plain`` option.

.. _`py namespaces`: index.html
.. _`py/__init__.py`: http://bitbucket.org/hpk42/py-trunk/src/trunk/py/__init__.py


Why a ``py.test`` instead of a ``pytest`` command?
++++++++++++++++++++++++++++++++++++++++++++++++++

Some of the reasons are historic, others are practical.  ``pytest``
used to be part of the ``py`` package which provided several developer
utilities, all starting with ``py.<TAB>``, thus providing nice
TAB-completion. If
you install ``pip install pycmd`` you get these tools from a separate
package.  These days the command line tool could be called ``pytest``
but since many people have gotten used to the old name and there
is another tool named "pytest" we just decided to stick with
``py.test`` for now.

pytest fixtures, parametrized tests
-------------------------------------------------------

.. _funcargs: funcargs.html

Is using pytest fixtures versus xUnit setup a style question?
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

For simple applications and for people experienced with nose_ or
unittest-style test setup using `xUnit style setup`_ probably
feels natural.  For larger test suites, parametrized testing
or setup of complex test resources using fixtures_ may feel more natural.
Moreover, fixtures are ideal for writing advanced test support
code (like e.g. the monkeypatch_, the tmpdir_ or capture_ fixtures)
because the support code can register setup/teardown functions
in a managed class/module/function scope.

.. _monkeypatch: monkeypatch.html
.. _tmpdir: tmpdir.html
.. _capture: capture.html
.. _fixtures: fixture.html

.. _`why pytest_pyfuncarg__ methods?`:

.. _`Convention over Configuration`: http://en.wikipedia.org/wiki/Convention_over_Configuration

Can I yield multiple values from a fixture function?
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

There are two conceptual reasons why yielding from a factory function
is not possible:

* If multiple factories yielded values there would
  be no natural place to determine the combination
  policy - in real-world examples some combinations
  often should not run.

* Calling factories for obtaining test function arguments
  is part of setting up and running a test.  At that
  point it is not possible to add new test calls to
  the test collection anymore.

However, with pytest-2.3 you can use the :ref:`@pytest.fixture` decorator
and specify ``params`` so that all tests depending on the factory-created
resource will run multiple times with different parameters.

You can also use the ``pytest_generate_tests`` hook to
implement the `parametrization scheme of your choice`_. See also
:ref:`paramexamples` for more examples.

.. _`parametrization scheme of your choice`: http://tetamap.wordpress.com/2009/05/13/parametrizing-python-tests-generalized/

pytest interaction with other packages
---------------------------------------------------

Issues with pytest, multiprocess and setuptools?
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++

On Windows the multiprocess package will instantiate sub processes
by pickling and thus implicitly re-import a lot of local modules.
Unfortunately, setuptools-0.6.11 does not ``if __name__=='__main__'``
protect its generated command line script.  This leads to infinite
recursion when running a test that instantiates Processes.

As of mid-2013, there shouldn't be a problem anymore when you
use the standard setuptools (note that distribute has been merged
back into setuptools which is now shipped directly with virtualenv).

.. include:: links.inc