What does the `in` operator in Python use for comparison?

Question

I am trying to understand what mechanism is used by in to compare the needle and the haystack.

[] in ([],[],[]) is True, so it cannot be is, because [] is [] is False
But math.nan in ([],[],math.nan) is True, too, so it cannot be ==, because math.nan==math.nan is False.

If it is neither == (comparison for equal value) nor is (comparison for object identity), what is it?

It's both. `__contains__` on standard collections uses both `is` and `==` (or a C equivalent to that). — khelwood, Jan 26 '21 at 20:51
It is essentially equivalent to the math/logic ```contains``` or ```∈```. It checks an iterable object to see if the value you are searching for is ```in``` the object. — goalie1998, Jan 26 '21 at 20:52
it uses both `is` and `==`, though `is` is an optimization. The assumption being that identity implies equality (which is not a safe assumption actually!) — juanpa.arrivillaga, Jan 26 '21 at 20:54
@goalie1998 kinda doesn't answer the question, the OP is asking "how is ∈ determined" — juanpa.arrivillaga, Jan 26 '21 at 20:55
Largely, if not precisely, a duplicate of [Checking for NaN presence in a container](https://stackoverflow.com/q/9904699/364696). — ShadowRanger, Jan 26 '21 at 20:55
@juanpa.arrivillaga The answer below suggests it is `or`, not `and`. — DYZ, Jan 26 '21 at 20:55
@DYZ yes, sorry, I didn't mean to imply that it check if something `is `and `==`, I meant it uses both. — juanpa.arrivillaga, Jan 26 '21 at 20:55
@juanpa.arrivillaga just giving examples OP might already be familiar with, which is why it's a comment, not an answer. — goalie1998, Jan 26 '21 at 20:56

score 3 · Accepted Answer · answered Jan 26 '21 at 20:51

3

As the docs say:

For container types such as list, tuple, set, frozenset, dict, or collections.deque, the expression x in y is equivalent to any(x is e or x == e for e in y).

So, it's both.

answered Jan 26 '21 at 20:51

bereal

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Anyway to see that using `dis.dis`? Or do you just need to know that when you see `(in)`, it is the implemented as any? – dfundako Jan 26 '21 at 20:52
@dfundako no, `is` is absolutely **not** implemented as `any`. And you couldn't see this with `dis.dis`, it will just show that you are using the binary operator `is`. It is *equivalent* to that. – juanpa.arrivillaga Jan 26 '21 at 20:54
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@dfundako internally, lists are implemented in C, so you can't see that in `dis`. For non-builtin containers, you can check `__contains__`. – bereal Jan 26 '21 at 20:55
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@dfundako: It's using [`PyObject_RichCompareBool`](https://docs.python.org/3/c-api/object.html#c.PyObject_RichCompareBool) deep in the C layer; you wouldn't see it with `dis`. That's a C optimized test that's explicitly documented to short-circuit for `==`/`!=` based on identity. – ShadowRanger Jan 26 '21 at 20:57

What does the `in` operator in Python use for comparison?

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