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Author abarnert
Recipients abarnert
Date 2015-12-26.22:26:42
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Serhiy Storchaka raised an issue ( with static type hints on #25864 (, which I believe also applies to runtime ABCs.

Consider this class, which uses `[]` in a way similar to generic types:

    class Thing:
        def __getitem__(self, specialization):
            return type(self)._specialize(specialization)(self)
        def __len__(self):
            return len(self._specializations)

Because this type looks like it supports the old-style sequence protocol, calling either `iter` or `reversed` on an instance will successfully return a useless iterator. (You may get an error when you start trying to iterate it, but you may not even then.) You don't want that, so you add either this:

        __iter__ = None
        __reversed__ = None

... or this:

        def __iter__(self): raise TypeError('not iterable')
        def __reversed__(self): raise TypeError('not iterable')

Unfortunately, doing either means that `issubclass(Thing,` now returns true. Which is the exact opposite of the intention of that check. (The same is true for `typing.Iterable` and `typing.Reversible`.) So, fixing the problem for duck typing creates the equivalent problem for explicit typing.

There are a few possible solutions here:

1. Maybe document it, otherwise do nothing.

2. Change the ABCs to check that the dunder method exists and is not None (or is callable, or is a non-data descriptor). Then, the one way to opt out is to assign `__iter__ = __reversed__ = None`.

3. Add an `ABC.unregister` method that can be used to explicitly state that this type does not support that ABC, regardless of what its `__subclasshook__` says.

Possible argument for #1: `Iterable` rarely has a problem. (Types that use `__getitem__` for something completely un-sequence-like, like `typing`'s generic types, usually don't have `__len__`. Types that have both `__getitem__` and `__len__`, like mappings, usually have a reasonable alternative `__iter__` to offer.) `Reversible` would have a problem if there was such an ABC, but there isn't. Off the top of my head, I can't think of any of the other implicit ABCs that are susceptible to this problem.

The counter-argument is that static typehinting definitely does have this problem (, and, depending on how that's solved, it may well make sense to use the same solution here.

If we do need a solution, #2 seems better than #3 (or anything else I could think up). The only problem there is that `def __iter__(self): raise TypeError('not iterable')` gives you a nicer error than `__iter__ = None`.
Date User Action Args
2015-12-26 22:26:42abarnertsetrecipients: + abarnert
2015-12-26 22:26:42abarnertsetmessageid: <>
2015-12-26 22:26:42abarnertlinkissue25958 messages
2015-12-26 22:26:42abarnertcreate