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[ty] Minor fixes to Protocol tests #20347

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AlexWaygood merged 2 commits into from
Sep 11, 2025
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[ty] Minor fixes to Protocol tests #20347

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[ty] Minor fixes to `Protocol` tests
AlexWaygood Sep 11, 2025
6ea485e
Update crates/ty_python_semantic/resources/mdtest/protocols.md
AlexWaygood Sep 11, 2025
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2 changes: 1 addition & 1 deletion crates/ty_python_semantic/resources/mdtest/expression/len.md
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Expand Up @@ -260,7 +260,7 @@ class SecondRequiredArgument:
def __len__(self, v: int) -> Literal[1]:
return 1

# TODO: Emit a diagnostic
# this is fine: the call succeeds at runtime since the second argument is optional
reveal_type(len(SecondOptionalArgument())) # revealed: Literal[0]
Comment on lines +263 to 264
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@AlexWaygood AlexWaygood Sep 11, 2025

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we could consider emitting a diagnostic on the definition of __len__ itself here (but that should be a disabled-by-default lint rule, IMO!), but I see no reason why we should emit a diagnostic at the len() call itself here. SecondOptionalArgument is a subtype of Sized.


# TODO: Emit a diagnostic
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2 changes: 1 addition & 1 deletion crates/ty_python_semantic/resources/mdtest/generics/legacy/functions.md
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Expand Up @@ -77,7 +77,7 @@ from typing import Protocol, TypeVar
T = TypeVar("T")

class CanIndex(Protocol[T]):
def __getitem__(self, index: int) -> T: ...
def __getitem__(self, index: int, /) -> T: ...
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@AlexWaygood AlexWaygood Sep 11, 2025

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The intent of this test is that list[str] should be a subtype of CanIndex; below, an instance of list[str] is passed into takes_in_protocol. But as written, list[str] is not a subtype of CanIndex, since all parameters of list.__getitem__ are positional-only


class ExplicitlyImplements(CanIndex[T]): ...

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2 changes: 1 addition & 1 deletion crates/ty_python_semantic/resources/mdtest/generics/pep695/functions.md
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Expand Up @@ -72,7 +72,7 @@ from typing import Protocol, TypeVar
S = TypeVar("S")

class CanIndex(Protocol[S]):
def __getitem__(self, index: int) -> S: ...
def __getitem__(self, index: int, /) -> S: ...

class ExplicitlyImplements[T](CanIndex[T]): ...

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25 changes: 22 additions & 3 deletions crates/ty_python_semantic/resources/mdtest/protocols.md
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Expand Up @@ -1813,9 +1813,28 @@ class Foo:
static_assert(not is_assignable_to(Foo, Iterable[Any]))
```

Because method members must always be available on the class, it is safe to access a method on
`type[P]`, where `P` is a protocol class, just like it is generally safe to access a method on
`type[C]` where `C` is a nominal class:
Because method members are always looked up on the meta-type of an object when testing assignability
and subtyping, we understand that `IterableClass` here is a subtype of `Iterable[int]` even though
`IterableClass.__iter__` has the wrong signature:

```py
from typing import Iterator, Iterable
from ty_extensions import static_assert, is_subtype_of, TypeOf

class Meta(type):
def __iter__(self) -> Iterator[int]:
yield from range(42)

class IterableClass(metaclass=Meta):
def __iter__(self) -> Iterator[str]:
yield from "abc"

static_assert(is_subtype_of(TypeOf[IterableClass], Iterable[int]))
```

Enforcing that members must always be available on the class also means that it is safe to access a
method on `type[P]`, where `P` is a protocol class, just like it is generally safe to access a
method on `type[C]` where `C` is a nominal class:

```py
from typing import Protocol
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