Spec/Conformance: subtyping callables with non-constant parameter mapping.

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Quiet
Tech stack
python
Domain
documentation, testing-qa

Research direction

Start with the callable assignability rules linked in the issue and reproduce the example with mypy, pyright, ty, and pyrefly. The issue names no repository files or conformance-test path; completion depends on deciding whether to clarify constant parameter mapping in the spec or add this case to conformance tests.

Written by the indexing model from the issue text.

Description

topic: typing spec

Consider the example below, which errors with all tested type checkers (mypy/pyright/ty/pyrefly):

from typing import Protocol

class Interval: ...

class Make(Protocol):
    def __call__(self, /, lower: float, upper: float) -> Interval: ...

def make_impl(
    string_or_lower: str | float | None = None,
    /,
    lower: float | None = None,
    upper: float | None = None,
) -> Interval: ...

def test() -> None:
    _f: Make = make_impl  # ❌️ type checkers error here.

I believe from a pure type theory POV, this assignment should be legal, because all legal arguments to Make are also legal arguments to make_impl. The spec phrases it in the same spirit:

A callable type B is assignable to a callable type A if the return type of B is assignable to the return type of A and the input signature of B accepts all possible combinations of arguments that the input signature of A accepts. All of the specific assignability rules described below derive from this general rule.

And I couldn't find anything else in https://typing.python.org/en/latest/spec/callables.html#assignability-rules-for-callables that would disallow this assignment.

It seems the type-checkers try to match the KEYWORD_OR_POSITIONAL parameters by name, which is incorrect. Make has 3 legal call signatures:

  1. Make(float, float)
  2. Make(float, upper=float)
  3. Make(lower=float, upper=float)

and all these 3 call signatures are supported by make_impl, but the parameter mapping is not constant:

  1. make_impl(float, float) -> {lower:string_or_lower, upper:lower}
  2. make_impl(float, upper=float) -> {lower:string_or_lower, upper:upper}
  3. make_impl(lower=float, upper=float) -> {lower:lower, upper:upper}

So either the spec should demand a constant parameter mapping, or this example should be added to the conformance tests.

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