Override Python's 'in' operator?

Question

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.}

Funderburk

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) – Tamikatamiko

Answer 1

MyClass.__contains__(self, item)

Farewell

Answer 2

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__.

Battaglia

Doesn't work for me :/ – Sheepherder

Answer 3

Another way of having desired logic is to implement __iter__.

If you don't overload __contains__ python would use __iter__ (if it's overloaded) to check whether or not your data structure contains specified value.

Calcaneus