What are the names of the magic methods for the operators "is" and "in"?

Question

I would like to make bool binary operations using the magic methods for these operators. For example, I can get a < b as getattr(a, '__lt__')(b) or a == b as getattr(a, '__eq__')(b).

Can I get a in b and a is b in such a way?

Fruit

Answer 1

in is __contains__ and is does not have a dunder method. I strongly suggest you use the functions in the operator module:

a < b  => operator.lt(a, b)
a == b => operator.eq(a, b)
a in b => operator.contains(a, b)
a is b => operator.is_(a, b)

Phira

Why would the operator module be preferable to the usual way? – Rhatany

It wouldn't be but if you need function versions of the operators the operator module is preferable to accessing dunder methods or writing your own wrappers. – Phira

I read getattr(a, '__lt__')(b) as asking for the function version of a < b which is operator.lt(a, b). You can pass around operator.lt. I saw no attempt at definition of a class in which you would define the dunder methods. – Phira

I agree with you now. Your answer definitely provides a superior alternative to using getattr. – Salzhauer

If you want operator.is_ with the first argument already filled in, like getattr(a, '__lt__') already has a as its first argument) you can use functools.partial. partial(operator.is_, a) is a function that only takes one argument. – Morphosis

Answer 2

For in, the correct dunder method is __contains__.

There is no method for is, because this is equivalent to id(a) == id(b). It compares the actual object ID used under the hood by Python, so is used to compare object identity, not object contents. Overwriting it within a class would break Python's object model, so it is not allowed.

Salzhauer

Answer 3

__contains__ is correct for in, with fall-back options if __contains__ isn't defined being __iter__ and __getitem__.

I am not really sure why you'd need to use getattr for is though; is is "defined" for every object in Python. There's no need to go through operator._is or (trying and failing) through getattr.

See the documentation on built-in types:

The behavior of the is and is not operators cannot be customized; also they can be applied to any two objects and never raise an exception.

^{(Emphasis mine)}

According to the snippets you provided, which just grab a function and call it using getattr(a, "function")(b), you already have the names of the objects you need to evaluate, just use is immediately; it is always available.

Demark