[1.12] Regression in is None narrowing

Bug Report

Regression in 1.12.

I'm not sure if this is deliberate or somehow got missed, but I thought aiohttp was in the mypy test suite, so should have been caught in the regression tests..

    def feed_data(
        self,
        data: bytes,
        SEP: Optional[_SEP] = None,
        *args: Any,
        **kwargs: Any,
    ) -> Tuple[List[Tuple[RawResponseMessage, StreamReader]], bool, bytes]:
        if SEP is None:
            SEP = b"\r\n" if DEBUG else b"\n"
        return super().feed_data(data, SEP, *args, **kwargs)

https://github.com/aio-libs/aiohttp/blob/01bfb4380742df87f910c2c62c619e0e9b9a0ed4/aiohttp/http_parser.py#L678-L684

Now results in:

aiohttp/http_parser.py:684:40: error: Argument 2 to "feed_data" of "HttpParser"
has incompatible type "Literal[b'\\r\\n', b'\\n'] | None"; expected
"Literal[b'\\r\\n', b'\\n']"  [arg-type]
            return super().feed_data(data, SEP, *args, **kwargs)
                                           ^~~

Expected Behavior

The variable can never be None at this point, so there should be no error (as in 1.11 and previous releases).

Your Environment

aio-libs/aiohttp#9527

Mypy configuration options: https://github.com/aio-libs/aiohttp/blob/master/.mypy.ini
Python version used: 3.11

@ilevkivskyi

This bisects to #17427. Here is a complete, slightly simplified reproducer:

from typing import Literal

SEP = Literal["a", "b"]

class Base:
    def feed_data(
        self,
        sep: SEP,
    ) -> int:
        return 0

class C(Base):
    def feed_data(
        self,
        sep: SEP | None = None,
    ) -> int:
        if sep is None:
            sep = reveal_type("a" if int() else "b")  # Union[Literal['a'], Literal['b']?]
            reveal_type(sep)  # None
        # Incompatible type "Literal['a', 'b'] | None"; expected "Literal['a', 'b']"
        return super().feed_data(sep)

Interestingly, we used to infer the correct union type before #17427, so this is perhaps not just a pre-existing issue that #17427 exposed but something more subtle.

cc @ilevkivskyi

Appears mypy_primer did catch it, but there were probably too many changes there for someone to notice the legitimate regression.

Yeah. I think the vast majority of new errors were legitimate.

Fix ternary union for literals (#18023)

Not sure this was fully resolved. The linked fix is in 1.14, but the aiohttp regression is still present:
https://github.com/aio-libs/aiohttp/actions/runs/12496766030/job/34868785013?pr=10222

@Dreamsorcerer

@Dreamsorcerer could you please find a simple self-contained repro? (I guess there may be something special about aiohttp setup) because the above example works for me in a recent master.

@ilevkivskyi

@ilevkivskyi Sorry for the delay, been a slow month.

I broke it down to a minimal reproducer, then realised that it's almost exactly the same as the above example. After some more testing I've realised that the reveal_type() is fixing the example. Simply removing the first reveal_type() reproduces the issue again:

from typing import Literal

SEP = Literal["a", "b"]

class Base:
    def feed_data(
        self,
        sep: SEP,
    ) -> int:
        return 0

class C(Base):
    def feed_data(
        self,
        sep: SEP | None = None,
    ) -> int:
        if sep is None:
            sep = "a" if int() else "b"  # Union[Literal['a'], Literal['b']?]
            reveal_type(sep)  # None
        # Incompatible type "Literal['a', 'b'] | None"; expected "Literal['a', 'b']"
        return super().feed_data(sep)

OK, nice, I have a fix, but it is too late, I will make a PR tomorrow.

@Dreamsorcerer

@Dreamsorcerer here is the real fix #18545

Navigation Menu

Repository navigation

[1.12] Regression in `is None` narrowing #18021

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

[1.12] Regression in is None narrowing #18021

Description

Activity

JukkaL commented on Oct 23, 2024

Dreamsorcerer commented on Oct 23, 2024

JukkaL commented on Oct 23, 2024

Dreamsorcerer commented on Dec 25, 2024

ilevkivskyi commented on Dec 27, 2024

Dreamsorcerer commented on Jan 26, 2025

ilevkivskyi commented on Jan 27, 2025

ilevkivskyi commented on Jan 27, 2025

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

[1.12] Regression in `is None` narrowing #18021