Python, provide type hints for a mixin that a property exists

Question

For example this mixin:

from lib import stringlib

class NiceNameMixin:
    @property
    def nice_first_name(self):
        return stringlib.clean_name(self.first_name)

    @property
    def nice_last_name(self):
        return stringlib.clean_name(self.last_name)

warns me:

Cannot access member "first_name" for type "NiceNameMixin" Member "first_name" is unknown.

What is the correct was of telling the type checker that the attributes self.first_name and self.last_name will exist, and it's okay?

Note I normally just # type: ignore these kind of things, but I am starting to think there must be a more suitable way.

Note: Using Pyright

Update: Class property example for @chepner

from rest_framework import serializers

class UtilsMixin:
    @classproperty
    def model_class(cls: serializers.ModelSerializer):  # Type of parameter Cls must be a supertype of its class "UtilsMixin"
        return cls.Meta.model  # Cannot access member "Meta" for type "ModelSerializer"   Member "Meta" is unknown

If the required attributes don't exist, do you want an error at runtime when calling the methods, or when inheriting from `NiceNameMixin`? — chepner, Oct 07 '22 at 12:33

score 3 · Answer 1 · answered Oct 07 '22 at 14:35

Adding to the excellent answer by @chepner. If one wants to express with types, that the child class has to implement the first_name and last_name attributes, consider annotating self with an appropriate typing.Protocol.

from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Protocol


class HasFirstAndLastName(Protocol):
    first_name: str
    last_name: str


class NiceNameMixin(ABC):
    @property
    def nice_first_name(self: HasFirstAndLastName) -> str:
        return self.first_name.capitalize()

    @property
    def nice_last_name(self: HasFirstAndLastName):
        return self.last_name.capitalize()


@dataclass
class User(NiceNameMixin):
    first_name: str
    last_name: str


@dataclass
class User2(NiceNameMixin):
    first_name: str


User("John", "Wick").nice_first_name  # OK
User2("John").nice_first_name  # Err

Alternatively, you can use `ABC` to declare `first_name` and `last_name` as abstract properties (attribute is definitely a drop-in replacement for property, being even wider, so `mypy` and other type checkers accept it too). — STerliakov, Oct 07 '22 at 14:45
Nice way to do it. It works for this example, I tried to use for a class property and it did nto work, please see my update — run_the_race, Oct 08 '22 at 07:39

chepner · Accepted Answer · 2022-10-07T12:27:28.173

You can simply add annotated, but uninitialized, names to the class:

class NiceNameMixin:
    first_name: str
    last_name: str

    @property
    def nice_first_name(self):
        return stringlib.clean_name(self.first_name)

    @property
    def nice_last_name(self):
        return stringlib.clean_name(self.last_name)

This does not guarantee that a class inheriting from NiceNameMixin will actually have those attributes, but it makes pyright happy with NiceNameMixin itself.

Python, provide type hints for a mixin that a property exists

Update: Class property example for @chepner

2 Answers2