Subclassing int for a revolving control number

[Peter Otten]

Chris Cioffi wrote:

> I'm trying to subclass int such that once it reaches a certain value,
> it flips back to the starting count.  This is for use as an X12
> control number.  Subclassing int and getting the display like I want
> it was trivial, however what's the best way to make it such that:
> 990 + 1 = 991
> 991 + 1 = 992
> ...
> 998 + 1 = 999
> 999 + 1 = 001
> 
> Should I set any of the special functions like __sub__ to raise
> NotImpemented and just make my own custom assignment and __add__
> functions?  (The other fucntions would make no sense in this
> context...)

OK, here's my take at the problem.

Peter


class IntN(int):
    """ Base class for int % N """
    def __new__(cls, value):
        value = value % cls.N
        if value < 0:
            value = cls.N - value
        return int.__new__(cls, value)
    def __add__(self, other):
        return self.__class__(int.__add__(self, other))
    def __sub__(self, other):
        return self.__class__(int.__sub__(self, other))

def makeNoimp(name):
    """ generate a method that raises a NotImplemented exception """
    def noimp(self, *args):
        raise NotImplementedError("method %s() not implemented" % name)
    return noimp

# shade the methods inherited from int
for name in "__mul__ __div__".split():
    setattr(IntN, name, makeNoimp(name))


class IntK(IntN):
    """ A weird way of implementing your (weird) specification """
    N = 999
    def __str__(self):
        return str(1 + self)
    __repr__ = __str__

if __name__ == "__main__":
    first = IntK(995)
    for i in range(10):
        print first,
        first += 1
    print

    first = IntK(5)
    for i in range(10):
        print first,
        first -= 1
    print

    try:
        first * 3
    except NotImplementedError, e:
        print e