Type hint for "can be compared" objects

Question

I am writing several functions that handle ordered datasets. Sometime, there is an argument that can be a int or float or a timestamp or anything that supports comparison (larger than / smaller than) and that I can use for trimming data for instance.

Is there a way to type-hint such a parameter? The typing module doesn't seem to include this, but is there some other way?

There's really no such thing as broad as "supports comparison". `int` values, for example, can be compared to other numbers, but not strings. — chepner, Aug 26 '22 at 16:59
I would do something like ComparableMixin (https://stackoverflow.com/questions/6907323/comparable-classes-in-python-3) and check for objects that inherit from it. But as @chepner wrote, I'm afraid this is not quite generic as in other languages e.g. C# — Lior Pollak, Aug 26 '22 at 17:00

Martijn Pieters · Accepted Answer · 2022-08-26T17:26:48.027

There is no standard 'comparable' ABC, no, as the rich comparison methods are really very flexible and don't necessarily return booleans.

The default built-in types return NotImplemented when applied to a type they can't be compared with, for example, while specialised libraries like SQLAlchemy and numpy use rich comparison methods to return completely different objects. See the documentation for the rich comparison methods for the details.

But you should be able to define a a Protocol subclass for specific expectations:

from typing import Protocol, TypeVar

T = TypeVar("T")

class Comparable(Protocol[T]):
    def __eq__(self: T, other: T) -> bool:
        ...

    def __lt__(self: T, other: T) -> bool:
        ...

    # ... etc

You may need to tweak the protocol to fit your exact expectations, and / or use a non-generic version that's specific to the types you use (perhaps with @overloaded definitions for specific types).

For sorting with the builtin sorted() function, __eq__ and __lt__ suffice.

Type hint for "can be compared" objects

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