Comparing class instances in python

Question

I built a new class in python which defines time with 6 figures (such as 18:45:00)

class Time(object):

    def __init__(self, hour, minute, second):
        minute = minute + second / 60
        hour = hour + minute / 60        
        self.hour = hour % 24
        self.minute = minute % 60
        self.second = second  % 60

I have also defined many methods to make it work as it should. the problem I have is with the cmp method:

def __cmp__(self,other):
    return cmp(self.to_seconds(),other.to_seconds())

It works fine when I try to compare times, if I'm sorting a list of times it also works fine. But if I'm trying to sort a list of times and integers or strings it also work. How can I define it to compare only times and to raise and error if trying to compare time with something that isn't.

Answer 1

You can use isinstance() to see if the argument is an instance of some class. See documentation .

Answer 2

You need to perform the type check in __cmp__ then act accordingly.

For example, maybe something like this:

import numbers

def __cmp__(self, other):
    other_seconds = None
    if hasattr(other, "to_seconds"):
        other_seconds = other.to_seconds()
    elif isinstance(other, numbers.Real):
        other_seconds = other

    if seconds is None:
        return NotImplemented

    return cmp(self.to_seconds(), seconds)

Answer 3

def __cmp__(self, other):
  if not isinstance(other, Time):
    return NotImplemented
  return cmp(self.to_seconds(), other.to_seconds())

NotImplemented is the constant to return for undefined comparison actions: http://docs.python.org/library/constants.html