Using python doctests.

sample.py

"""
An example of using doctest to automatically test.

One function that adds items together.

>>> adder(1,2)
3
"""

def adder(x,y):
    """Return the result of adding x and y
    >>> adder(1,2)
    3
    >>> adder(a,b)
    Traceback (most recent call last):
      ...
    NameError: name 'a' is not defined
    >>> adder("a","b")
    'ab'
    """
    result = x + y
    return result

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