def fact(n):    """    Calculate 1*2*...*n    >>> fact(1)    1    >>> fact(10)    3628800    >>> fact(-1)    Traceback (most recent call last):        ...    ValueError    """    if n < 1:        raise ValueError()    if n == 1:        return 1    return n*fact(n-1)if __name__ == '__main__':    import doctest    doctest.testmod(verbose=True)