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Add hint for AsyncIterator incompatible return type #15883

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Aug 30, 2023
Merged

Add hint for AsyncIterator incompatible return type #15883

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Add hint for AsyncIterator incompatible return type
ikonst Aug 16, 2023
ef2f8a7
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ikonst Aug 16, 2023
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16 changes: 16 additions & 0 deletions mypy/messages.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1310,6 +1310,22 @@ def return_type_incompatible_with_supertype(
code=codes.OVERRIDE,
)

original = get_proper_type(original)
override = get_proper_type(override)
if (
isinstance(original, Instance)
and isinstance(override, Instance)
and override.type.fullname == "typing.AsyncIterator"
and original.type.fullname == "typing.Coroutine"
and len(original.args) == 3
and original.args[2] == override
):
self.note(f'Consider declaring "{name}" in {target} without "async"', context)
self.note(
"See https://mypy.readthedocs.io/en/stable/more_types.html#asynchronous-iterators",
context,
)

def override_target(self, name: str, name_in_super: str, supertype: str) -> str:
target = f'supertype "{supertype}"'
if name_in_super != name:
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16 changes: 16 additions & 0 deletions test-data/unit/check-async-await.test
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Expand Up @@ -1021,3 +1021,19 @@ def coro() -> Generator[int, None, None]:
reveal_type(coro) # N: Revealed type is "def () -> typing.AwaitableGenerator[builtins.int, None, None, typing.Generator[builtins.int, None, None]]"
[builtins fixtures/async_await.pyi]
[typing fixtures/typing-async.pyi]

[case asyncIteratorInProtocol]
from typing import AsyncIterator, Protocol

class P(Protocol):
async def launch(self) -> AsyncIterator[int]:
raise BaseException

class Launcher(P):
def launch(self) -> AsyncIterator[int]: # E: Return type "AsyncIterator[int]" of "launch" incompatible with return type "Coroutine[Any, Any, AsyncIterator[int]]" in supertype "P" \
# N: Consider declaring "launch" in supertype "P" without "async" \
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Shouldn't it instead say: "consider adding async to def lauch()"?

Because in my experience, AsyncIterators are way more common with async def rather than regular def

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@ikonst ikonst Aug 29, 2023 •
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Ehh, this is confusing :|

The 'problem' here should really be reported on this line:

  class P(Protocol):
!>    async def launch(self) -> AsyncIterator[int]:
         raise BaseException

but we don't put notices proactively on declarations that smell funny, or do we?

The error on the derived Launcher.launch gives us the excuse to make a notice. What we want to say is "listen, we must error on Launcher.launch, but Launcher.launch is probably fine, consider changing the base declaration instead".

# N: See https://mypy.readthedocs.io/en/stable/more_types.html#asynchronous-iterators
raise BaseException

[builtins fixtures/async_await.pyi]
[typing fixtures/typing-async.pyi]