[1.16 regression] a cast is both needed and redundant at the same time

@Tinche

Bug Report

Since 1.16, structlog's package type check fails with:

src/structlog/stdlib.py:1160: error: Redundant cast to "MutableMapping[str, Any]"  [redundant-cast]
                ed = p(logger, meth_name, cast(EventDict, ed))
                                          ^
Found 1 error in 1 file (checked 1 source file)

If I remove the cast, I get:

src/structlog/stdlib.py:1160: error: Argument 3 has incompatible type "Mapping[str, Any] | str | bytes | bytearray | tuple[Any, ...]"; expected
"MutableMapping[str, Any]"  [arg-type]
                ed = p(logger, meth_name, ed)
                                          ^~
Found 1 error in 1 file (checked 1 source file)

If I add a reveal_type, I get on both 1.15 & 1.16:

src/structlog/stdlib.py:1160: note: Revealed type is "typing.MutableMapping[builtins.str, Any]"
src/structlog/stdlib.py:1160: note: Revealed type is "Union[typing.Mapping[builtins.str, Any], builtins.str, builtins.bytes, builtins.bytearray, builtins.tuple[Any, ...]]"

So I guess Mypy got somehow smarter/confused about that?

To Reproduce

git clone git@github.com:hynek/structlog.git
git checkout d948027
run tox -e mypy-pkg

I really tried to create a minimal reproducer, but given that I don't quite understand what's going on, I've failed.

The relevant code is here: https://github.com/hynek/structlog/blob/e941e337d312eaf256595d9d09da4542c5e94d0b/src/structlog/stdlib.py#L1160

The types are defined here: https://github.com/hynek/structlog/blob/e941e337d312eaf256595d9d09da4542c5e94d0b/src/structlog/typing.py#L61-L76

Expected Behavior

I would expect it to keep working, or at least decide whether or not I need the cast. 🤓

Actual Behavior

I don't seem to find a way to make the code pass except using Any all over the place I guess.

Your Environment

Mypy version used: 1.16
Mypy command-line flags: none
Mypy configuration options from mypy.ini (and other config files): https://github.com/hynek/structlog/blob/e941e337d312eaf256595d9d09da4542c5e94d0b/pyproject.toml#L175-L191
Python version used: 3.13

edit I managed to work around it with the help of @Tinche but it still looks like I managed to break poor little Mypy.

Thanks! Probably proximal cause is #18588

If you'd like to upgrade to 1.16, p(logger, meth_name, cast(Any, ed)) is probably a foolproof workaround

Minimised repro:

from typing import Callable, cast

class X: ...

ProcessorReturnValue = X | str | bytes
Processor = Callable[[X], ProcessorReturnValue]

def main_cast(p: Processor) -> None:
    ed: ProcessorReturnValue
    ed = cast(X, ...)

    for _ in range(5):
        ed = p(cast(X, ed))

def main_no_cast(p: Processor) -> None:
    ed: ProcessorReturnValue
    ed = cast(X, ...)

    for _ in range(5):
        ed = p(ed)

I guess the cast is redundant the second time through the loop...

Yes, I hope we just have to handle codes.REDUNDANT_CAST the same way as codes.REDUNDANT_EXPR in #19118. Maybe there are more such candidates? (UNUSED_IGNORE or so)

Avoid false `unreachable`, `redundant-expr`, and `redundant-casts` wa…

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[1.16 regression] a cast is both needed and redundant at the same time #19170

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[1.16 regression] a cast is both needed and redundant at the same time #19170

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hauntsaninja commented on May 30, 2025

hauntsaninja commented on May 30, 2025

tyralla commented on May 30, 2025

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