Python issue

coolman0stress · 05-10-2004, 03:58 PM

Ok, i'm playing around with writing custom class comparison methods by implementing my own __cmp__ methods. I'm encountering a strange issue that i not entirely "getting":

class:

Code:

class Person:
    def __init__(self, name, *arg):
        self.name = name

    def __cmp__(self, other):
        print "comparing %s to %s = %s" % (self.name, other.name, self.name == other.name)
        return self.name == other.name

    def __repr__(self):
        return "Person:\n\tname: %s" % (self.name)

output:

Code:

>>> import comparing as c
>>> p1 = c.Person("John")
>>> p2 = c.Person("Bob")
>>> p1 == p2
comparing John to Bob = False
True

Why does the method return 'True' when the comparison (as is shown in the print string) is actually 'False'?

The_Nerd · 05-10-2004, 04:05 PM

Ummm.... I could be wrong about this (because though I know Python, I am no guru), shouldn't those be intagers instead of strings? Like follows:

Code:

def __cmp__(self, other):
        print "comparing %s to %s = %d" % (self.name, other.name, int(self.name == other.name))
        return int(self.name == other.name)

Tell me if that works.

coolman0stress · 05-10-2004, 04:09 PM

I tried that, but would give the same result.

Even tried this:

Code:

    def __cmp__(self, other):
        print "comparing %s to %s = %s" % (self.name, other.name, self.name == other.name)
        ret = int(self.name == other.name)
        print ret
        return ret

Output:

Code:

>>> p1 == p2
comparing John to Bob = False
0
True

There must be something i'm missing in my understanding of Python.

rshaw · 05-10-2004, 04:33 PM

could just do a 'p1 is p2' instead of p1 == p2, return blah

coolman0stress · 05-10-2004, 06:21 PM

is would compare references, i'm actually trying to compare what's inside the objects. So even if the references are different, the data contained within the objects could be the same...

Strike · 05-11-2004, 02:40 AM

Quote:

__cmp__ (self, other)
Called by comparison operations if rich comparison (see above) is not defined. Should return a negative integer if self < other, zero if self == other, a positive integer if self > other. If no __cmp__(), __eq__() or __ne__() operation is defined, class instances are compared by object identity (``address''). See also the description of __hash__() for some important notes on creating objects which support custom comparison operations and are usable as dictionary keys. (Note: the restriction that exceptions are not propagated by __cmp__() has been removed since Python 1.5.)

With __cmp__, returning 0 means "are equal", returning something < 0 means "less than", and returning something > 0 means "greater than". So, returning self.name == other.name would return 0 for unequal names, but that would be interpreted as "are equal". You can easily just return self.name.__cmp__(other.name) just to make it consistent

coolman0stress · 05-11-2004, 10:08 AM

Humm... that sort of makes sense.

Thanks Strike

Strike · 05-11-2004, 01:35 PM

Well, generally if you are just deferring to another method, I generally like to invoke it the same way. So if I've got a __str__ method that basically just returns the str representation of some member of that class, I'd do:

Code:

def __str__(self):
    return self.item.__str__()

I guess that comes as habit since when you subclass things and you want to invoke the parent class constructors, you do:

Code:

class Child(Parent):
    def __init__(self, *args, **kwargs):
        # stuff here
        Parent.__init__(self, args, kwargs)