Brute-Forceing keys in __getitem__

[LukeGrainwalker]

Bug report

Bug description:

I have encountered a problem when using the __getitem__ method in cpython 3.10.12 and 3.15.0a0:
When a class has a __getitem__ but not a __contains__ method python seems to brute force the key when using the keyword in. I know this must be intentional (brute forcing does not happen by accident). But in the code, i encountered this in, i was using strings as keys, which python will never brute force. In this case it will never return. From a users perspective it would be more helpful to have a clear error message instead.

class test:
    def __init__(self):
        pass
    def __getitem__(self, key):
        print(key)

if "this is a test" in test():
    print("it is in it?")

this will count up to infinity!

I hope this will not drown among the 7000 Issues.
Thanks for reviewing this and God Bless you!

CPython versions tested on:

3.15, CPython main branch, 3.10

Operating systems tested on:

Linux

[StanFromIreland]

This is technically the correct behavior, but it does seem wrong, from the docs:

For user-defined classes which do not define __contains__() [...] the old-style iteration protocol is tried: if a class defines __getitem__(), x in y is True if and only if there is a non-negative integer index i such that x is y[i] or x == y[i], and no lower integer index raises the IndexError exception. (If any other exception is raised, it is as if in raised that exception).

[zware]

This is not "brute-forcing", but rather the documented membership protocol. A class with a __getitem__ method that never raises IndexError and no __contains__ method is broken for the purposes of containment.

[LukeGrainwalker]

yes i should have read the documentation, sorry for bothering. so in the case that the key is not found i am expected to raise the IndexError. Thanks for explaining it.