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Allow unpacking from TypeVars with iterable bounds #13425

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merged 3 commits into from
Aug 19, 2022

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@tdsmith tdsmith commented Aug 15, 2022

Description

TypeVars aren't iterable, but their bounds might be! Resolve a TypeVar to its bounds before trying to decide how to unpack one of its instances. This allows unpacking from instances of TypeVars with iterable bounds, which otherwise fails.

Fixes #13402.

Test Plan

Added a test for a minimal reproducer.

TypeVars aren't iterable, but their bounds might be!
Resolve a TypeVar to its bounds before trying to decide how to unpack
one of its instances.

Fixes python#13402.
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from typing import Tuple, TypeVar
TupleT = TypeVar("TupleT", bound=Tuple[int, ...])
def f(t: TupleT) -> None:
a, *b = t
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Let's add reveal_type(a) and reveal_type(b) here. This would catch some unexpected types.

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Done!

@@ -3168,6 +3168,9 @@ def check_multi_assignment(
# TODO: maybe elsewhere; redundant.
rvalue_type = get_proper_type(rv_type or self.expr_checker.accept(rvalue))

if isinstance(rvalue_type, TypeVarType):
rvalue_type = get_proper_type(rvalue_type.upper_bound)
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What if there's no upper bound? 🤔 Let's add a test case for this as well!

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That's a good thing to check 😅 It looks like it does the right thing!

I left the first test case in check-bound.test and added the new test case to check-typevar-unbound.test but let me know if they should be organized differently.

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Thank you!

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tdsmith commented Aug 17, 2022

Thanks for your help!

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According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

@sobolevn sobolevn merged commit 7d95e2e into python:master Aug 19, 2022
@tdsmith tdsmith deleted the iterable-typevar-unpacking branch August 22, 2022 15:24
hauntsaninja pushed a commit to hauntsaninja/mypy that referenced this pull request Sep 10, 2022
* Allow unpacking from TypeVars by resolving bounds

TypeVars aren't iterable, but their bounds might be!
Resolve a TypeVar to its bounds before trying to decide how to unpack
one of its instances.

Fixes python#13402.
hauntsaninja added a commit that referenced this pull request Sep 12, 2022
…13425) (#13644)

TypeVars aren't iterable, but their bounds might be! Resolve a TypeVar
to its bounds before trying to decide how to unpack one of its
instances.

Fixes #13402

Co-authored-by: Tim D. Smith <github@tim-smith.us>
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Instances of type variables bounded by an iterable type support iteration but not unpacking
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