All instances of types.GenericAlias are iterable on Python 3.11+

@mrahtz

All generic aliases appear to be iterable on Python 3.11+, meaning you can do some fairly bizarre things that don't really make any sense:

>>> list(dict[int, int])
[*dict[int, int]]
>>> import collections.abc
>>> list(collections.abc.Generator[int, str, bytes])
[*collections.abc.Generator[int, str, bytes]]

This is due to the implementation of PEP 646 that was added in #31019 by @mrahtz. It was a deliberate decision at the time: 4aa94df is the commit in that PR branch that removes the restriction that only tuple can be unpacked (the commit was done in response to a review comment here: #31019 (comment)). However, there wasn't much discussion at the time.

Do we want to stick with this behaviour, on the basis that the runtime usually prefers to be lenient when it comes to typing constructs, leaving it to type checkers to point out invalid usage? Or should this be considered a bug?

the runtime usually prefers to be lenient when it comes to typing constructs, leaving it to type checkers to point out invalid usage

I vote for this option!

My thinking at the time was that this could be useful for allowing something like tuple[*Iterable[int]], or something related to the TypeVarDict idea that was recently brought up on typing-sig. I don't see a need to forbid this again, especially since we've already had a series of releases allowing it.

It's probably not really a big deal, but one impact of this is that there's yet another way in which collections.abc.Callable and typing.Callable are slightly different from each other:

>>> import collections.abc, typing
>>> list(collections.abc.Callable[..., int])
[*collections.abc.Callable[..., int]]
>>> list(typing.Callable[..., int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '_CallableGenericAlias' object is not iterable

@AlexWaygood

@AlexWaygood yes, this happens because most of the typing constructs directly inherit from

class _NotIterable:
    """Mixin to prevent iteration, without being compatible with Iterable.

    That is, we could do:
        def __iter__(self): raise TypeError()
    But this would make users of this mixin duck type-compatible with
    collections.abc.Iterable - isinstance(foo, Iterable) would be True.

    Luckily, we can instead prevent iteration by setting __iter__ to None, which
    is treated specially.
    """

    __slots__ = ()
    __iter__ = None

Seems like there's little appetite for changing this

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All instances of `types.GenericAlias` are iterable on Python 3.11+ #103450

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All instances of types.GenericAlias are iterable on Python 3.11+ #103450

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hauntsaninja commented on Apr 11, 2023

JelleZijlstra commented on Apr 11, 2023

AlexWaygood commented on Apr 11, 2023

sobolevn commented on Apr 12, 2023

AlexWaygood commented on Apr 29, 2023

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All instances of `types.GenericAlias` are iterable on Python 3.11+ #103450