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[spec] overload subtyping rules are too strict.

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#2,021 3 comentarios 2 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
5/5
Tiempo estimado
Más de una semana
Aptitud para principiantes
35/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Estancado
Stack tecnológico
python
Área
compilers

Línea de trabajo

Comienza con las reglas de subtipado de overloads en la sección de callables de la especificación de typing y revisa la discusión relacionada en typing/discussions/1782. Compara las reglas indicadas con el ejemplo IntScalarOurs e IntScalarTheirs y determina después si la especificación necesita una revisión fundamentada en principios o una regla aclarada de condición suficiente.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

topic: other

The typing spec states that, when A and B are potentially overloaded methods

If a callable B is overloaded with two or more signatures, it is assignable to callable A if at least one of the overloaded signatures in B is assignable to A

If a callable A is overloaded with two or more signatures, callable B is assignable to A if B is assignable to all of the signatures in A

However, this definition seems to be too strict. Consider the following example:

from typing import Protocol, Self, overload

class IntScalarOurs(Protocol):
    def __add__(self, other: int | Self, /) -> Self: ...

class IntScalarTheirs(Protocol):
    @overload
    def __add__(self, other: int, /) -> Self: ...
    @overload
    def __add__(self, other: Self, /) -> Self: ...

These two protocols specify the exact same runtime behavior. Yet, following the rules of the spec, IntScalarOurs is assignable to IntScalarTheirs, but IntScalarTheirs is not assignable to IntScalarOurs.

Case 1: A=IntScalarOurs, B=IntScalarTheirs.

If a callable B is overloaded with two or more signatures, it is assignable to callable A if at least one of the overloaded signatures in B is assignable to A

  • Is (self, int) -> Self assignable to (self, int | Self) -> Self? No, because int is not a supertype of int | Self (contravariance)
  • Is (self, Self) -> Self assignable to (self, int | Self) -> Self? No, because Self is not a supertype of int | Self (contravariance)

Thus, IntScalarTheirs is not assignable to IntScalarOurs.

Case 2: A=IntScalarTheirs, B=IntScalarOurs.

If a callable A is overloaded with two or more signatures, callable B is assignable to A if B is assignable to all of the signatures in A

  • Is (self, int | Self) -> Self assignable to (self, int) -> Self? Yes.
  • Is (self, int | Self) -> Self assignable to (self, Self) -> Self? Yes.

Thus, IntScalarOurs is assignable to IntScalarTheirs.

From a pure set-theoretic POV, it seems that subtyping functions should follow a very simply rule based on the domain.

$f <: g$ if and only if $\text{dom}(f) ⊇ \text{dom}(g)$ and $f(x) <: g(x)$ for all $x∈\text{dom}(g)$

What is currently specced appears to be a lemma for a sufficient condition, but not a necessary one.
Individual type-checkers can of course use such lemmas if the general case is too difficult to check/implement (and communicate that to their users), but shouldn't the spec be biased towards the theoretically principled point of view, when applicable?

Related Discussions

https://github.com/python/typing/discussions/1782

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