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:
    ...

Solution 1

MyClass.__contains__(self, item)

Solution 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__.

Solution 3

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/

Comments

[Tomasz Gandor]

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)

[Peter Hansen]

@pthulin, yours may be "more complete" in terms of code, but Ignacio's links to the documentation, which is always a big plus for some.

[Zoran Pavlovic]

@PEter. YEah, but some of us prefer a nice, visual representation of the answer. Ignacio's did little to benefit the question other than direct us here first instead of google, no thank you.

[Peter Hansen]

Zoran, I agree, and I even upvoted this answer and not the other. I'm just pointing out that a truly good answer should always link to docs, if available.

[demongolem]

All, links die and that is why Ignacio's answer is shaky on SO. Links + example is the best and that is why a combination of the two answers we are talking about is best.

[wizzwizz4]

@demongolem The official Python documentation will die with or after the interpreter becomes undownloadable. And it isn't a link-only answer; I came to this page looking for the answer and found it without clicking on any further links (i.e. the prototype was sufficient). I do agree in principle, but imho that doesn't apply here.