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Use Union for tuple fallback? #7616
Description
- Please insert below the code you are checking with mypy,
from typing import Iterable, Iterator, Union
from typing_extensions import Protocol
class MixedIter(Protocol):
def __iter__(self) -> Iterator[Union[int, str]]: ...
def f(xs: Iterable[Union[int, str]]) -> str:
return ''.join(str(x) for x in xs)
def g(xs: MixedIter) -> str:
return ''.join(str(x) for x in xs)
mixedTuple = (56, 'A')
reveal_type(mixedTuple)
reveal_type(mixedTuple.__iter__)
print(f(mixedTuple))
print(g(mixedTuple))- What is the actual behavior/output?
mixedtuple.py:14: note: Revealed type is 'Tuple[builtins.int, builtins.str]'
mixedtuple.py:15: note: Revealed type is 'def () -> typing.Iterator[builtins.object*]'
mixedtuple.py:17: error: Argument 1 to "g" has incompatible type "Tuple[int, str]"; expected "MixedIter" [arg-type]
- What is the behavior/output you expect?
I'd expect both f(mixedTuple) and g(mixedTuple) to be accepted without errors.
- What are the versions of mypy and Python you are using?
mypy 0.730 on Python 3.7.2.
Activity
[-]Use Union return type for Tuple.__iter__ for mixed-type tuples[/-][+]Use Union for tuple fallback?[/+]msullivan commentedon Oct 5, 2019
The situation here is that the use of it as an
Iterableis enabled as part of a bevy of unfortunate special cases in our subtype checker: https://github.com/python/mypy/blob/master/mypy/subtypes.py#L277-L286.The type used to figure out things like the type of
mixedTuple.__iter__is the tuple's "fallback type", which is computed using a join: https://github.com/python/mypy/blob/master/mypy/typeops.py#L34It seems possible that we could switch to using a union for tuple fallbacks instead of a join and drop those special cases subtyping special cases. The normal worry about switching from using a join to using a union--that it will break calls functions with invariant type constructors--seems less pronounced here since tuples are immutable.
I think the main concern would be when lists are created from tuples with
list(mixedTuple). But constraints also has its own batch of special casing (https://github.com/python/mypy/blob/master/mypy/constraints.py#L366-L370) that seems to mitigate that...hauntsaninja commentedon Jul 3, 2024
Fixed by #17408