Override Python's 'in' operator?

232
votes

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:
    ...
3
pythonoperator-overloadingoperatorsin-operator
I was actually searching how to override the is and is not operators. Like a query = tinydb.Query().field == value, to also be able to write Query().field is not None. But it seems I'm left with __eq__ and __ne__ for the time being, which leads to the unpythonic Query().field != None. (sarc) - Tomasz Gandor

3 Answers

231
votes

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__.

7
votes

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/