django - simple equation

Question

why does model.diff return 18446744073709551615 in template, when model is like this and model.pos is 0 and model.neg is 1?:

class Kaart(models.Model):
    neg = models.PositiveIntegerField(default=0)
    pos = models.PositiveIntegerField(default=0)
    def diff(self):
     return self.pos - self.neg

Answer 1

You're getting underflow, where self.pos - self.neg should give -1, but you have a positive field, so it wraps around and you get 0 - 1 = 18446744073709551615, the largest positive number representable by PositiveIntegerField.

For your reference, 18446744073709551615 = 2**64 - 1, which means that PositiveIntegerField stores 64-bit values.

Answer 2

Peter's probably right. Try:

def diff(self):
    return int(self.pos) - int(self.neg)

That should do the trick.