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.
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 plugin which allows to return deferreds from pytest-style tests, allowing to use pytest fixtures: explicit, modular, scalable and other features.
In 2012, some work is going into the pytest-django plugin. 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.
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 assert feedback. 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 --assertmode=off option.
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 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.
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.
There are two conceptual reasons why yielding from a factory function is not possible:
However, with pytest-2.3 you can use the Fixtures as Function arguments decorator and specify params so that all tests depending on the factory-created resource will run multiple times with different parameters.
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 middle 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).