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Design by contract on python vanilla (Python recipe) by Alan Cristhian Ruiz
ActiveState Code (http://code.activestate.com/recipes/578767/)

The code below is an example that shows that we can do design by contract in python without any non-standard module.

Python, 30 lines
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def function(a, b):
    # description and test
    """Adds an integer and a float and returns a str.
    >>> function(1, 3.0)
    '4.0'
    """

    # precondition
    assert isinstance(a, int), 'a argument should be int'
    assert isinstance(b, float), 'b argument should be float'

    #implementation
    try:
        c = str(a + b)
        return c

    # postcondition
    finally:
        assert isinstance(c, str), 'c variable should be str'


# run all docstring test on debug mode
if __debug__:
    import doctest
    doctest.testmod()


if __name__ == '__main__':
    # do something
    pass

This pattern can be used in function definitions, class definitions, modules and in the same places where you can use documentation strings, try/finally statement and assert statement.

Tags: demo, design_pattern, productivity

2 comments

Alan Cristhian Ruiz (author) 10 years, 5 months ago  # | flag

Note that this code is not a good example of using assertions. I recommend reading the tutorial When to use assert made ​​by Steven D'Aprano.

Sam Denton 10 years, 4 months ago  # | flag

Did you read the tutorial that you referenced? "assertions should be used for: [...] * checking contracts (e.g. pre-conditions and post-conditions);" I would say that this is a good example of using assertions.