Some Issues and Questions
If you don't find an answer here, you may checkout
`pytest Q&A at Stackoverflow <http://stackoverflow.com/search?q=pytest>`_
or other :ref:`contact channels` to get help.
On naming, nosetests, licensing and magic
How does py.test relate to nose and unittest?
py.test 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 py.test. nose_ was originally created
as a clone of ``py.test`` when py.test 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 py.test relate to twisted's trial?
Since some time py.test 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 to
return deferreds from pytest-style tests, allowing to use
:ref:`fixtures` and other features.
how does py.test 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 to use all pytest features_ most of which
are not available from Django directly.
.. _features: features.html
What's this "magic" with py.test? (historic notes)
Around 2007 (version ``0.8``) some people thought that py.test
was using too much "magic". It had been part of the `pylib`_ which
contains a lot of unreleated 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 relese 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, py.test explicitely 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 loosing
assertions in test modules.
py.test contains a second mostly obsolete assert debugging technique,
invoked via ``--assert=reinterpret``, activated by default on
Python-2.5: When an ``assert`` statement fails, py.test 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
.. _`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. ``py.test``
used to be part of the ``py`` package which provided several developer
utilities, all starting with ``py.<TAB>``, thus providing nice
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.
Function arguments, parametrized tests and setup
.. _funcargs: funcargs.html
Is using funcarg- 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 funcargs_ may feel more natural.
Moreover, funcargs are ideal for writing advanced test support
code (like e.g. the monkeypatch_, the tmpdir_ or capture_ funcargs)
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
.. _`why pytest_pyfuncarg__ methods?`:
Why the ``pytest_funcarg__*`` name for funcarg factories?
We like `Convention over Configuration`_ and didn't see much point
in allowing a more flexible or abstract mechanism. Moreover,
it is nice to be able to search for ``pytest_funcarg__MYARG`` in
source code and safely find all factory functions for
the ``MYARG`` function argument.
With pytest-2.3 you can use the :ref:`@pytest.fixture` decorator
to mark a function as a fixture function.
.. _`Convention over Configuration`: http://en.wikipedia.org/wiki/Convention_over_Configuration
Can I yield multiple values from a fixture function function?
There are two conceptual reasons why yielding from a factory function
is not possible:
* 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.
* 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.
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`_.
.. _`pytest_generate_tests`: test/funcargs.html#parametrizing-tests
.. _`parametrization scheme of your choice`: http://tetamap.wordpress.com/2009/05/13/parametrizing-python-tests-generalized/
py.test interaction with other packages
Issues with py.test, 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.
A good solution is to `install Distribute`_ as a drop-in replacement
for setuptools and then re-install ``pytest``. Otherwise you could
fix the script that is created by setuptools by inserting an
``if __name__ == '__main__'``. Or you can create a "pytest.py"
script with this content and invoke that with the python version::
if __name__ == '__main__':
.. _`install distribute`: http://pypi.python.org/pypi/distribute#installation-instructions
.. include:: links.inc