Python unit testing with doctest module

The doctest module is part of the Python standard library. There is no need to write separate test functions/methods in doctest, you simply copy the expected results and paste them in the docstring that corresponds to the tested function.

class Counter:
    def __init__(self, value = 0):
        """
        >>> Counter().value
        0
        >>> Counter(100).value
        100
        """
        self.value = value
    
    def add(self, x):
        """
        >>> c = Counter()
        >>> c.add(5)
        5
        >>> c.value
        5
        >>> c.add(0)
        Traceback (most recent call last):
        ...
        ValueError
        >>> c.skip_me # doctest: +SKIP
        """
        if not x:
            raise ValueError
        self.value += x
        return self.value

if __name__ == '__main__':
    import doctest
    doctest.testmod()

In order to execute all tests just run the following (on success no output is given):

python doctestexample.py

Alternatively you can run doctest with unittest:

if __name__ == '__main__':
    import doctest, unittest
    suite = unittest.TestSuite()
    suite.addTest(doctest.DocTestSuite(__name__))
    runner = unittest.TextTestRunner()
    runner.run(suite)

Read more about doctest here.