Doctest for factorial function.

fact.py

def fact(n):
    """
    Computes factorial.

    >>> fact(1)
    1
    >>> fact(5)
    120
    >>> fact(-1)    # doctest: +IGNORE_EXCEPTION_DETAIL
    Traceback (most recent call last):
        ...
    ArithmeticError: ...

    """
    if n > 1:
        return n * fact(n - 1)
    elif n == 1 or n == 0:
        return 1
    else:
        raise(ArithmeticError("Undefined for negative numbers"))

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