python / python/mypy

annotations on `self` are not understood with `Protocol`s

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Description

Bug Report

When interpreting a given type as a given Protocol, annotations on self in the Protocol are apparently ignored.

To Reproduce

mypy-play.net



from __future__ import annotations

from typing import *



                          #━━━━━━━━━━━━━━━━━#
                          #  EXISTING CODE  #
                          #━━━━━━━━━━━━━━━━━#


class ConcreteClass:

    # here 'transform()' is used, but we might use '__neg__()' or
    # '__invert__()' in a similar way (so many built-in/third-party
    # classes would qualify as 'TransformAware')

    def transform(self) -> int:
        return id(self)



                           #━━━━━━━━━━━━━━━━#
                           #  LIBRARY CODE  #
                           #━━━━━━━━━━━━━━━━#

# this can be changed


Ico = TypeVar('Ico', covariant=True)        # input, covariant
Oco = TypeVar('Oco', covariant=True)        # output, covariant


class TransformAware(Protocol[Ico, Oco]):

    def transform(self: Ico) -> Oco: ...



                      #━━━━━━━━━━━━━━━━━━━━━━━━━#
                      #  DEMONSTRATE PROBLEMS:  #
                      #━━━━━━━━━━━━━━━━━━━━━━━━━#


I = TypeVar('I')                            # input
O = TypeVar('O')                            # output


def interpret_transform_aware(_: Type[TransformAware[I, O]]) -> Tuple[I, O]:
    i: I
    o: O
    return i, o


reveal_type(interpret_transform_aware(ConcreteClass))
        # actual output:
        #   note: Revealed type is "Tuple[<nothing>, builtins.int]"
        # expected output:
        #   note: Revealed type is "Tuple[ConcreteClass, int]"



                           #───────────────#
                           #  MOTIVATION:  #
                           #───────────────#


class TransformDriver(Generic[I, O]):

    def __init__(self, klass: Type[TransformAware[I, O]]): ...

    def transform_value(self, value: I) -> O:
        use_value = cast(TransformAware[I, O], value)
        return use_value.transform()

Expected Behavior

(See: inline comments.)

Interpreting ConcreteClass as TransformAware should correctly infer both type variables.

Actual Behavior

(See: inline comments.)

Mypy gives the type of Ico as <nothing>.

Your Environment

  • Mypy version used: 1.6.1
  • Mypy command-line flags: (none necessary)
  • Mypy configuration options from mypy.ini (and other config files): (none necessary)
  • Python version used: 3.8, 3.11

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the mypy-play.net reproducer and the Protocol handling code; the payload names no repository file or test. Trace how annotations on self are used when interpreting ConcreteClass as TransformAware, then add coverage showing that both type variables are inferred as ConcreteClass and int.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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