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Allow unpacking from TypeVars with iterable bounds #13425

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merged 3 commits into from Aug 19, 2022
Merged

Allow unpacking from TypeVars with iterable bounds #13425

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7df6183
Allow unpacking from TypeVars by resolving bounds
tdsmith Aug 15, 2022
2224a5a
Reveal types of a and b
tdsmith Aug 17, 2022
529c571
Add test for unbounded typevar
tdsmith Aug 17, 2022
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3 changes: 3 additions & 0 deletions mypy/checker.py
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Expand Up @@ -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.


if isinstance(rvalue_type, UnionType):
# If this is an Optional type in non-strict Optional code, unwrap it.
relevant_items = rvalue_type.relevant_items()
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10 changes: 10 additions & 0 deletions test-data/unit/check-bound.test
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Expand Up @@ -215,3 +215,13 @@ if int():
b = 'a' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
twice(a) # E: Value of type variable "T" of "twice" cannot be "int"
[builtins fixtures/args.pyi]


[case testIterableBoundUnpacking]
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!

reveal_type(a) # N: Revealed type is "builtins.int"
reveal_type(b) # N: Revealed type is "builtins.list[builtins.int]"
[builtins fixtures/tuple.pyi]