How to: Override Equals in Python

February 6, 2012

Just got bit by this. So if you want to override the equality operator in python the simplest thing to do is override the __eq__ method. For example:

class MyFoo(object):

def __init__(self, equalityprop):

self.equalityprop = equalityprop

def __eq__(self, obj):

return isinstance(obj, MyFoo) and obj.equalityprop == self.equalityprop

Then you can use the == operator and it behaves as you would expect.

MyFoo(“bar”) == MyFoo(“bop”) # False

MyFoo(“bar”) == MyFoo(“bar”) # True

What you might not realize is that this has absolutely no effect on the != operator.

MyFoo(“bar”) == MyFoo(“bop”) # True

MyFoo(“bar”) != MyFoo(“bar”) # True (!)

If you want to override != (which you probably do) you also have to override the __ne__ method. One simple thing you can do is:

def __ne__(self, obj):

return not self == obj

Good times.

Chief Accelerator