Constraints on generic class with multiple TypeVar's

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Feature
Clarity
Needs clarification
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

The issue provides a Python reproducer but names no source file or test. Start by running the example with mypy and locating the TypeVar, Generic, and overload-checking tests. Done means establishing whether paired constraints can be expressed and, if supported, adding coverage for the allowed and rejected combinations.

Written by the indexing model from the issue text.

Description

feature priority-2-low

Is there a way to limit the possible combinations of TypeVar's instantiations for Generics with multiple TypeVar's?

Let's look at the following example:

import sys
from typing import TypeVar, Generic, overload

assert sys.version_info >= (3, 5)


class Foo(object):
    def __init__(self, dummy):
        self.dummy = dummy


class Bar(object):
    def __init__(self, dummy):
        self.dummy = dummy


class Apple(object):
    def __init__(self, dummy):
        self.dummy = dummy


class Pear(object):
    def __init__(self, dummy):
        self.dummy = dummy


Var1 = TypeVar('Var1', Foo, Apple)
Var2 = TypeVar('Var2', Bar, Pear)


@overload
def combine(left: Foo, right: Bar) -> None:
    ...


@overload
def combine(left: Apple, right: Pear) -> None:
    ...


def combine(left, right):
    print(left.dummy + " and " + right.dummy)


class MyClass(Generic[Var1, Var2]):
    def __init__(self, a: Var1) -> None:
        self.dummy = dummy  # type: Var1

    def add(self, other: Var2) -> None:
        combine(self.dummy, other)

Here, mypy produces this error output:

mypy_example.py:50: error: No overload variant of "combine" matches argument types [mypy_example.Foo*, mypy_example.Pear*]
mypy_example.py:50: error: No overload variant of "combine" matches argument types [mypy_example.Apple*, mypy_example.Bar*]

Is there any possibility to restrict MyClass to MyClass[Apple, Pear] and MyClass[Foo, Bar] but disallow MyClass[Apple, Bar] and MyClass[Foo, Pear]? This is for python 3.5.

Dominant language
Python
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