Question

If I have a list of objects, I can use the __cmp__ method to override objects are compared. This affects how the == operator works, and the item in list function. However, it doesn’t seem to affect the item in set function – I’m wondering how I can change the MyClass object so that I can override the behaviour how the set compares items.

For example, I would like to create an object that returns True in the three print statements at the bottom. At the moment, the last print statement returns False.

class MyClass(object):
    def __init__(self, s):
        self.s = s
    def __cmp__(self, other):
        return cmp(self.s, other.s)

instance1, instance2 = MyClass("a"), MyClass("a")

print instance2==instance1             # True
print instance2 in [instance1]         # True
print instance2 in set([instance1])    # False

Answer 1

set uses __hash__ for comparison. Override that, and you’ll be good:

class MyClass(object):
    def __init__(self, s):
        self.s = s
    def __cmp__(self, other):
        return cmp(self.s, other.s)
    def __hash__(self):
        return hash(self.s) # Use default hash for 'self.s'

instance1, instance2 = MyClass("a"), MyClass("a")
instance3 = MyClass("b")

print instance2==instance1             # True
print instance2 in [instance1]         # True
print instance2 in set([instance1])    # True