Some Issues and Questions


This FAQ is here only mostly for historic reasons. Checkout pytest Q&A at Stackoverflow for many questions and answers related to pytest and/or use 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 plugin which allows you to return deferreds from pytest-style tests, allowing the use of pytest fixtures: explicit, modular, scalable and other features.

how does pytest work with Django?

In 2012, some work is going into the pytest-django plugin. It substitutes the usage of Django’s test and allows the use of all pytest features most of which are not available from Django directly.

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 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 losing assertions in test modules.

pytest contains a second, mostly obsolete, assert debugging technique, invoked via --assert=reinterpret, activated by default on Python-2.5: 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 --assertmode=off option.

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

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.

Can I yield multiple values from a fixture function 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 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.

You can also use the pytest_generate_tests hook to implement the parametrization scheme of your choice.

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).