Mypy allows calling `in` keyword on `Iterable`

[tgavoille]

Bug Report

Mypy doesn't raise an error when checking if an element is in an Iterable while it does when directly using __contains__.

To Reproduce

from typing import Any, Iterable

def iterable_contains(iterable: Iterable[Any], element: Any):
    if element in iterable:  # No error but I thingk it should raise
        print(element)
    if iterable.__contains__(element):  # Error, expected
        print(element)

Expected Behavior

Mypy should raise an error on the first if statement

Actual Behavior

But it doesn't

Your Environment

Found this with earlier python and mypy versions but example is reproducible with :

• Mypy version used: 1.7.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.0

[AlexWaygood]

I think mypy's behaviour is correct here?

>>> class OnlyHasIter:
...     data = range(10)
...     def __iter__(self): yield from self.data
...
>>> 1 in OnlyHasIter()
True
>>> 11 in OnlyHasIter()
False
>>> 'foo' in OnlyHasIter()
False
>>> OnlyHasIter().__contains__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'OnlyHasIter' object has no attribute '__contains__'

[tgavoille]

Yes it seems so and actually runtime implementation states that __contains__ will try membership using __iter__ then __getitem__. I'm sorry I didn't saw this before, probably would have been better as a discussion.

Could you point me to some documentation about how in might try other dunders than __contains__ and why direct call to __contains__ should fail if object implements __iter__

[AlexWaygood]

Sure, it's documented here: https://docs.python.org/3/reference/expressions.html#membership-test-details

[tgavoille]

Thank you