Comparable types with mypy

Question

I'm trying to create a generic class to express that a value has lower and upper bounds, and to enforce those bounds.

from typing import Any, Optional, TypeVar

T = TypeVar("T")

class Bounded(object):
    def __init__(self, minValue: T, maxValue: T) -> None:
        assert minValue <= maxValue
        self.__minValue = minValue
        self.__maxValue = maxValue

However, mypy complains that:

error: Unsupported left operand type for <= ("T")

Apparently typing module doesn't allow me to express this (although it looks like adding Comparable might happen in the future).

I think it would be enough to check that object has __eq__ and __lt__ methods (for my use case at least). Is there any way to currently express this requirement in Python so that Mypy would understand it?

Answer 1

After a bit more research, I found a solution: Protocols. Since they are not fully stable (yet of Python 3.6), they have to be imported from the typing_extensions modules.

import typing
from typing import Any
from typing_extensions import Protocol
from abc import abstractmethod

C = typing.TypeVar("C", bound="Comparable")

class Comparable(Protocol):
    @abstractmethod
    def __eq__(self, other: Any) -> bool:
        pass

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

    def __gt__(self: C, other: C) -> bool:
        return (not self < other) and self != other

    def __le__(self: C, other: C) -> bool:
        return self < other or self == other

    def __ge__(self: C, other: C) -> bool:
        return (not self < other)

Now we can define our type as:

C = typing.TypeVar("C", bound=Comparable)

class Bounded(object):
    def __init__(self, minValue: C, maxValue: C) -> None:
        assert minValue <= maxValue
        self.__minValue = minValue
        self.__maxValue = maxValue

And Mypy is happy:

from functools import total_ordering

@total_ordering
class Test(object):
    def __init__(self, value):
        self.value = value
    def __eq__(self, other):
        return self.value == other.value
    def __lt__(self, other):
        return self.value < other.value

FBounded(Test(1), Test(10))
FBounded(1, 10)