Override Python's 'in' operator?

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:
    ...

_{See also What does __contains__ do, what can call __contains__ function for the corresponding question about what __contains__ does.}

MyClass.__contains__(self, item)