Overriding a class to be left hand side of in operator

Question

Apparently the __contains__ special method allows to implement the in evaulation when the object with the method is on the right hand side of it. I have a piece of code where the in must be implemented by the left hand operand. How do I go about it?

Answer 1

I was able to get this to work by overriding __eq__.

Consider the code "value in some_list". It will iterate through some_list until it finds value. But how does it know if it found value? By comparing it. That's what you want to override.

class Twenty(object):
    def __init__(self):
        self.x = 20
            
    def __eq__(self, y):
        print "comparing", self.x, "to", y
        return self.x == y
    
value = Twenty()
assert value in [10, 20, 30]
assert value not in [1, 2, 3]

output

comparing 20 to 10
comparing 20 to 20
comparing 20 to 1
comparing 20 to 2
comparing 20 to 3