Toolbox for testing Python

It is always easy and fun to do something, if you have right tools. Writing tests is not exception. Here is my toolbox, all things at one place. I hope, the following text will save somebody’s time and Google’s bandwidth.

Here we go.


It is a meta tool, which tests code using PyFlakes and pep8. The first one is a static analyzer and the second one is a code style checker. They can be used separately, but I prefer they work as a team. It helps to find stupid errors such as unused variables or imports, typos in names, undefined variables, and so on. It also helps to keep code consistent according to PEP 8 Style Guide for Python Code, which is critical for code-style nazis like me. The usage is quite simple:

$ flake8 coolproject
coolproject/ F401 'shutil' imported but unused
coolproject/ E225 missing whitespace around operato
coolproject/ F811 redefinition of function 'readlines' from line 723
coolproject/ F841 local variable 'errors' is assigned to but never used

Additionally, Flake8 includes complexity checker, but I never use it. However, it can help to decrease WTFs per minute during code review, I guess.


It is a unit-test framework, an extension of traditional unittest. I never use the last one itself, so I cannot adequately compare it with Nose. However, at a glance, Nose-based tests is more readable and compact. But it is only my subjective opinion.

Another benefit of Nose is its plugins. Some of them I use from time to time, but there are two, which I use unconditionally on each my project: doctest and cover.

The doctest plugin collects test scenarios from source code doc-strings and run them using Doctest library. It helps to keep doc-strings consistent with the code they describe. It also good place for unit tests for simple functions and classes. If test cases are not too complex, it will be enough to cover the code directly in the doc-string.

The cover plugin calculates test coverage and generates reports like this one:

Name                      Stmts   Miss  Cover   Missing
coolproject                  20      4    80%   33-35, 39
coolproject.module           56      6    89%   17-23
TOTAL                        76     10    87%

Such reports help to check test cases themselves and significantly improve the quality of ones. The cover plugin uses Coverage tool behind the scene, so you have to manually install it.

Nose is perfectly integrated with Setuptools. By the way, that is another reason to use the last one. I prefer to store Nose settings in the setup.cfg file, which usually looks like this:


It makes Nose usage very simple:

$ nosetests
tests.test1 ... ok
tests.test2 ... ok
tests.test3 ... ok
Doctest: coolproject.function1 ... ok
Doctest: coolproject.module.function2 ... ok

Name                      Stmts   Miss  Cover   Missing
coolproject                  20      4    80%   33-35, 39
coolproject.module           56      6    89%   17-23
TOTAL                        76     10    87%
Ran 5 tests in 0.021s



There is no way to write tests without mocks. At the most cases, Mock library is all what you need. However, there are other useful libraries, that can be helpful in particular cases.

  • Venusian can help to mock decorated functions deferring decorator action on the separate step.
  • FreezeGun is a neat mock for date and time. There is nothing you cannot do using Mock library, but it has already been done. So, just use it.
  • Responses is a mock for Requests library. If you develop client for third-party REST-service using Requests, that is what you need.

Additionally, I strongly recommend to look over the perfect article Python Mock Gotchas by Alex Marandon.


Tox gets things together and makes them run against different Python versions. It is like a command center for all testing infrastructure. It automatically creates virtual environments for specified Python versions, installs test dependencies and runs tests. And all of these is done using single command tox.

For example, tox.ini described bellow sets up testing for Python 3.3, 3.4, and PyPy, using Nose for unit tests and Flake8 for static analysis of source code of project itself, as well as source code of unit tests.


    flake8 coolproject
    flake8 tests

The usage of this tool is not limited by tests only. But it deserves a separate article, so I will write it soon.


I am pretty sure the list above is not complete. And there is a lot of awesome testing libraries that make life easier. So, post your links in the comments. I will try to keep the article updated.