Step 7 - Automate the tests

Welcome to step 7 in your training as a scientific Python code ninja: test automation.

Tests are extremely useful, as long as you run them. Unfortunately, a common pattern in many projects is this:

  1. Tests are written and run by the person who wrote them.
  2. Others, and even the test-writer at a future date, forget to run the tests before committing new code. Or they decide not to run the tests because they forget how, or they don’t want to wait for the tests to finish.
  3. The new code breaks the tests. By the time somebody notices, it’s too much trouble to go back and figure out where things went wrong, much less how to fix them.

How do you avoid this? You make running the tests dead simple and fully automatic. This step is about making the tests run with a single command; step 8 is about having a machine run them whenever you commit code, without any action on your part.

As a matter of fact, in Step 5 we set things up so that the tests in one file could by run with a single command. But eventually, you will have tests in multiple files. Besides doctests, you may write additional tests. If you need to type 10, or even 3 commands to run them all, there will be a mental barrier and eventually you’ll get lazy.


There are a number of test automation tools for Python, but to keep things simple I will focus on the one I know best: nose. Nose does one simple thing: it automatically finds all the tests in your project and runs them. How does it know where the tests are? Well, it can easily recognize doctests, and it will also pick up any function whose name contains “test” and that is in file whose name contains “test”.

What to do:

As usual, this should only take two minutes.

  1. In case it isn’t installed already, do pip install nose.
  2. First, go to your project directory and type nosetests --with-doctest. The nose executable is named “nosetests” and the option --with-doctest tells it to also run any doctests it finds. You should get a message saying that some number of tests were run, with an “OK” at the end.
  3. To see how nosetests becomes really useful, either: (a) add a doctest in another file; or (b) add a file called containing at least one function with “test” in the name. The test function should check something and raise an exception if things are wrong. See this example in my demo project.
  4. Now type nosetests --with-doctest again. You should see that your new tests were also run. To get more detailed output, try nosetests --with-doctest --vs.

That’s it. Now, if you add more doctests or other tests, you can run them all just by typing nosetests --with-doctest. That’s the kind of thing you might want to add to your README. Your rule now should be:

Always run the tests before you commit code.

Note: this post was originally written by David Ketcheson and posted at

under the Creative Commons Attribution 3.0 Unported License.

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