How can I hint that a type is comparable with typing

Question

Say I want to write a comparison sorting function, I can hint that the input must be a sequence with Sequence[T] (or MutableSequence[T] in this cas

Answer 1

This version works with the current mypy version.

Based on the thread in the typing repo: https://github.com/python/typing/issues/59

from __future__ import annotations

from abc import abstractmethod
from typing import MutableSequence, Protocol, TypeVar


class Comparable(Protocol):
    """Protocol for annotating comparable types."""

    @abstractmethod
    def __lt__(self: CT, other: CT) -> bool:
        pass


CT = TypeVar("CT", bound=Comparable)

def comparison_sort(s: MutableSequence[CT]) -> None:
    pass


comparison_sort([1, 2, 3])  # OK
comparison_sort([1.0, 2.0, 3.0])  # OK
comparison_sort(["42", "420", "2137"])  # OK
comparison_sort([1, 2, "3"])  # mypy error 

link to Github gist.