Is Python magical? Consider the trivial unit test below:

import unittest

class TestSomething(unittest.TestCase):
  def test1(self):
    self.assert_(True)

if __name__ == '__main__':
  unittest.main()

Unless you’re a Python guru, I’m sure you’ve wondered how unittest.main() found and ran TestSomething and TestSomething.test1. If you look at the unittest source code, you find a nice trick:

# unittest.py

class TestProgram:
  def __init__(...):
    ...
    self.runTests()

main = TestProgram

Ah, unittest.main isn’t a function, it’s a class! So unittest.main() creates a new TestProgram object, whose initializer then goes ahead and runs the unit tests for you. But how does TestProgram know to load the TestSomething class and run TestSomething.test1?

Most of this work is done by the unittest.TestLoader class. TestLoader will look at the current module (i.e. your current file), examine each class in it, and load classes that inherit unittest.TestCase. Then, TestLoader will look inside these classes for methods that have test as their prefix.

After this, TestProgram will know what tests it needs to run.

Of course, unittest is more complicated than this, but this describes the main flow of unittest. I suggest that you download the Python source code and read unittest.py yourself. It’s a lot of fun, really. Plus, reading source code is a great way to improve and enrich your coding, and Python’s source is one of the best treasures around.

Archived comments; imported from Posterous

3530 views and 1 response

Oct 15 2011, 1:52 PM

Steffen Hoffmann responded:

Thanks for sharing this - exactly what I was looking for (before looking into the code itself). As soon as one has got a clue about Python in general, I agree strongly that the code, commented or not, is a valuable resource. ’