class FactorialError(Exception): pass
class NegativeNumberError(FactorialError): pass
class NotAnIntegerError(FactorialError): pass

def factorial(number):
    """find the factorial of an integer."""
    if number < 0:
        raise NegativeNumberError, 'Negative Number has no factorial'
    if int(number) <> number:
        raise NotAnIntegerError, 'Only intergers have factorial'
    if number == 0:
        return 1
    else:
        return number * factorial(number - 1)