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Use normalized tuples for fallback calculation #19111

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Merged
merged 6 commits into from
Jul 22, 2025
Merged

Use normalized tuples for fallback calculation #19111

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9b6f908
Normalize tuples before calculating the fallback
sterliakov May 19, 2025
3989212
Merge remote-tracking branch 'upstream/master' into bugfix/gh-19105-t…
sterliakov Jul 4, 2025
bebdf28
Maybe this?
sterliakov Jul 4, 2025
0aaa011
Fix selfcheck
sterliakov Jul 4, 2025
8bb25fa
And a test
sterliakov Jul 4, 2025
03a0a5b
Merge branch 'master' into bugfix/gh-19105-trivial-unpack-in-base-cla…
sterliakov Jul 21, 2025
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3 changes: 2 additions & 1 deletion mypy/semanal_shared.py
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Original file line number Diff line number Diff line change
Expand Up @@ -46,6 +46,7 @@
TypeVarLikeType,
TypeVarTupleType,
UnpackType,
flatten_nested_tuples,
get_proper_type,
)

Expand Down Expand Up @@ -290,7 +291,7 @@ def calculate_tuple_fallback(typ: TupleType) -> None:
fallback = typ.partial_fallback
assert fallback.type.fullname == "builtins.tuple"
items = []
for item in typ.items:
for item in flatten_nested_tuples(typ.items):
# TODO: this duplicates some logic in typeops.tuple_fallback().
if isinstance(item, UnpackType):
unpacked_type = get_proper_type(item.type)
Expand Down
6 changes: 6 additions & 0 deletions mypy/semanal_typeargs.py
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Expand Up @@ -102,6 +102,8 @@ def visit_type_alias_type(self, t: TypeAliasType) -> None:
# If there was already an error for the alias itself, there is no point in checking
# the expansion, most likely it will result in the same kind of error.
get_proper_type(t).accept(self)
if t.alias is not None:
t.alias.accept(self)
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This looks like a duplicate work, did you try deleting the accept just above? (if you delete it, don't forget to update the comment)

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That breaks one test:

[case testValidTypeAliasValues]
from typing import TypeVar, Generic, List

T = TypeVar("T", int, str)
S = TypeVar("S", int, bytes)

class C(Generic[T]): ...
class D(C[S]): ...  # E: Invalid type argument value for "C"

U = TypeVar("U")
A = List[C[U]]
x: A[bytes]  # E: Value of type variable "T" of "C" cannot be "bytes"

V = TypeVar("V", bound=int)
class E(Generic[V]): ...
B = List[E[U]]
y: B[str]  # E: Type argument "str" of "E" must be a subtype of "int"

Removing this line makes both errors on alias instantiation disappear. That's quite expected since get_proper_type calls _expand_once, so it is not equivalent to simply accepting the target alias - we do not want to ignore args here. We can limit that to only accepting if args are non-empty, but I don't think this can result in any significant speedup.

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OK, this is kind of unfortunate. We already accept args in isolation at the very start (see super call), then we accept expansion, and finally the target in isolation. But I guess this is the price to pay for having both:

  • Support for unrestricted type vars in alias definitions
  • Doing some in-place patching in this visitor


def visit_tuple_type(self, t: TupleType) -> None:
t.items = flatten_nested_tuples(t.items)
Expand Down Expand Up @@ -254,6 +256,10 @@ def visit_unpack_type(self, typ: UnpackType) -> None:
def check_type_var_values(
self, name: str, actuals: list[Type], arg_name: str, valids: list[Type], context: Context
) -> bool:
if self.in_type_alias_expr:
# See testValidTypeAliasValues - we do not enforce typevar compatibility
# at the definition site. We check instantiation validity later.
return False
is_error = False
for actual in get_proper_types(actuals):
# We skip UnboundType here, since they may appear in defn.bases,
Expand Down
27 changes: 27 additions & 0 deletions test-data/unit/check-typevar-tuple.test
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Expand Up @@ -2666,3 +2666,30 @@ def identity(smth: _FT) -> _FT:
class S(tuple[Unpack[Ts]], Generic[T, Unpack[Ts]]):
def f(self, x: T, /) -> T: ...
[builtins fixtures/tuple.pyi]

[case testNoCrashSubclassingTupleWithTrivialUnpack]
# https://github.com/python/mypy/issues/19105
from typing import Unpack

class A(tuple[Unpack[tuple[int]]]): ...
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Could you please also add a test for

class C(tuple[Unpack[tuple[int, ...]]]): ...

(unless there is already such test case)? I am not sure we properly handle such types. Also maybe test not just class creation, but some tuple ops as well, such as e.g. unpacking etc.

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Surprisingly we don't have such a test too, but it was already working before my patch - *tuple[int, ...] is not a trivial unpack. But your feeling is overall correct - I'm able to make it crash with

a: A
tuple(a)

and also get a terrible diagnostic on

b: B
_, = b  # E: Variadic tuple unpacking requires a star target

despite B clearly not being variadic. I'll move this to draft for now, there's little benefit from merging a crash fix that causes a next one immediately...

class B(tuple[Unpack[tuple[()]]]): ...

a: A
reveal_type(tuple(a)) # N: Revealed type is "builtins.tuple[builtins.int, ...]"
(x,) = a

b: B
(_,) = b # E: Need more than 0 values to unpack (1 expected)
[builtins fixtures/tuple.pyi]

[case testNoCrashSubclassingTupleWithVariadicUnpack]
# https://github.com/python/mypy/issues/19105
from typing import Unpack

class A(tuple[Unpack[tuple[int, ...]]]): ...

a: A
tuple(a)
(x,) = a
(_,) = a
[builtins fixtures/tuple.pyi]
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