Override Python's 'in' operator?

MyClass.__contains__(self, item)

A more complete answer is:

class MyClass(object):

    def __init__(self):
        self.numbers = [1,2,3,4,54]

    def __contains__(self, key):
        return key in self.numbers

Here you would get True when asking if 54 was in m:

>>> m = MyClass()
>>> 54 in m
True  

See documentation on overloading __contains__.

You might also want to take a look at an infix operator override framework I was able to use to create a domain-specific language:

http://code.activestate.com/recipes/384122/