Python list remove method: how's the implementation?

Question

In java, I have my client class that have the "code" attr, and the equals method. Method equals receives another client and compares with itself's code attr.

In python, I just read that we have the __cmp__ method, to do the same as java method equals. Ok, I did that. I created my class client, with "code" attr and the method comp that verify if the code is the same.

class Client():
    def __init__(self, code):
        self.code = code

    def __cmp__(self, obj):
        return obj.code == self.code

    def __repr__(self):
        return str(self.code)

Then I put 3 Client objects in a python's list:

bla = [Client(1), Client(2), Client(3)]

Then, when I try:

bla.remove(Client(3))

The amazing python removes the first element (the Client with code 1).

What I am doing wrong? I searched the implementation of list in python's Lib files, but is not easy to find.

Anyone can help?

Answer 1

http://docs.python.org/reference/datamodel.html#object.__cmp__

__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.

Basically, you should change your implementation of __cmp__ to be...

def __cmp__(self, obj):
    return cmp(obj.code, self.code)

The cmp() builtin function of Python is specifically designed to return the values that __cmp__ is expected to return by comparing its two arguments.

There is also a different function in Python called __eq__ which only checks equality, for which your current implementation of __cmp__ would be better suited.

Answer 2

Sounds like you actually want __eq__

class Client():
    def __init__(self, code):
        self.code = code

    def __eq__(self, obj):
        return obj.code == self.code

    # this is how you usually write cmp, Amber explained the problem
    def __cmp__(self, other):
        return cmp(self.code, other.code)

    def __repr__(self):
        return str(self.code)

Btw, what happens in your buggy example is that __cmp__ returns False as expected. But in Python False == 0 and returning 0 from __cmp__ means the compared elements are equal. So that is why it removes the first element!

Answer 3

Yeah, I just saw that. comp must return a numeric value. got it. SOLVED. Amber, nice one. I'll use cmp() built-in.