If I am creating my own class in Python, what function should I define so as to allow the use of the in operator, e.g.

class MyClass(object):

m = MyClass()

if 54 in m:

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

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: