[issue30352] The 'in' syntax should work with any object that implements __iter__

[Edouard KLEIN]

New submission from Edouard KLEIN:

In this StackOverflow question:

http://stackoverflow.com/questions/43935187/how-come-an-object-that-implements-iter-is-not-recognized-as-iterable/43935360#43935360

the question of why an object that implements __iter__ through __getattr__ does not work with the "in" syntax is raised. The accepted answer is that the interpreter checks for __iter__ instead of just calling it.

If that's the case, I think that:
- Either the behaviour of the interpreter should be changed to accept any object that implements __iter__, regardless of how,
- Or, if there is a technical reason why the interpreter can't change its behavior, the documentation https://docs.python.org/3/library/stdtypes.html#iterator-types should make it clear that __iter__ must be hard coded into the object.

Here is the code of the object that arguably should be iterable but is not:

class IterOrNotIter:
    def __init__(self):
        self.f = open('/tmp/toto.txt')
    def __getattr__(self, item):
        try:
            return self.__getattribute__(item)
        except AttributeError:
            return self.f.__getattribute__(item)

IterOrNotIter().__iter__().__next__()  # Works
'a' in IterOrNotIter()  # Raises a TypeError

----------
components: Interpreter Core
messages: 293547
nosy: Edouard KLEIN
priority: normal
severity: normal
status: open
title: The 'in' syntax should work with any object that implements __iter__
type: behavior
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30352>
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