It seems that mypy is having problems taking into account all the superclasses and reports missing attributes. Here is a simple example:

import uuid

from flask_sqlalchemy import SQLAlchemy
from sqlalchemy import String
from sqlalchemy.dialects.postgresql import UUID
from sqlalchemy.orm import mapped_column, DeclarativeBase


db = SQLAlchemy()


class Base(DeclarativeBase):
    pass


class IdMixin:
    id = mapped_column(UUID(as_uuid=True), nullable=False, default=uuid.uuid4, primary_key=True)


class ImageMixin:
    image_name = mapped_column(String, default=None)
    image_uri = mapped_column(String, default=None)


class Model(IdMixin, Base):
    pass


class Post(Model, ImageMixin):
    content = mapped_column(String)


class Comment(Model, ImageMixin):
    text = mapped_column(String)


def generate_image_uri(image_name: str) -> str:
    return f"https://example.com/{image_name}"


models = [Comment, Post]
for model in models:
    entities = db.session.query(model).all()
    for entity in entities:
        entity.image_uri = generate_image_uri(image_name=entity.image_name) # Line 45

Mypy reports:

mypy-warning.py:45: error: "Model" has no attribute "image_uri"  [attr-defined]
mypy-warning.py:45: error: "Model" has no attribute "image_name"  [attr-defined]

It works just fine when I move the ImageMixin to the Model class and then only subclass Model in the Post and Comment classes, but this is not what I want as I don’t necessarily want all models to mix in the image functionality.