Mypy and Inheriting from a class that is an attribute on an instance

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EDIT

The issue arises when trying to inherit from a class that is an attribute of an instance. This mcve repros it, I'll leave the rest of the question below for posterity:

class A:
    class SubA:
        pass


a = A()

class B(a.SubA):
    pass

mypy output:

Name 'a.SubA' is not defined

This passes:

class A:
    class SubA:
        pass


class B(A.SubA):
    pass

The example in this Related Issue is pretty much exactly what Flask-SQLAlchemy does to provide the declarative base class under the db namespace. In the issue, mypy maintainer asserts that they wouldn't support the pattern.

My question is, what is incorrect about the above pattern such that mypy would not support it? Especially in the context that it is used by a large project such as Flask-SQLAlchemy.

Further, what is the best way for users of Flask-SQLAlchemy and mypy to manage this in their projects?

ORIGINAL QUESTION

This question isn't about the lack of Flask-SQLAlchemy stubs. Accepting that, I came across this question.

Please help me to understand why the following does not work as I expect.

In my environment I have only Flask-SQLAlchemy and mypy installed.

I have mypy configured with ignore_missing_imports = True.

A mypy pass over the following:

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()


class Widget(db.Model):
    id = db.Column(db.Integer, primary_key=True)

reveals:

error: Name 'db.Model' is not defined

So, I've attempted to subclass SQLAlchemy to provide an annotation for Model, which shows in __annotations__ on the object, yet the mypy analysis of the module doesn't change:

from flask_sqlalchemy import SQLAlchemy
from flask_sqlalchemy.model import DefaultMeta


class TypedSQLAlchemy(SQLAlchemy):
    Model: DefaultMeta


db = TypedSQLAlchemy()

print(db.__annotations__)  # {'Model': <class 'flask_sqlalchemy.model.DefaultMeta'>}


class Widget(db.Model):
    id = db.Column(db.Integer, primary_key=True)

When I execute the file, the print(db.__annotations__) command displays {'Model': <class 'flask_sqlalchemy.model.DefaultMeta'>}, yet still mypy has the same error:

error: Name 'db.Model' is not defined

I would expect that providing an annotation for db.Model should make that error go away.

Earlier Edit

I've initially misinterpreted the error as it doesn't suggest the attribute Model doesn't exist on db, it suggests that the name db.Model doesn't exist in the namespace. But why would it treat db.Model as a full name, and not db as the name defined locally and Model as it's attribute? Is this something to do with trying to inheriting from a class variable?

Also, my annotation was incorrect, should be:

class TypedSQLAlchemy(SQLAlchemy):
    Model: Type[DefaultMeta]
python-typing flask-sqlalchemy mypy typechecking python
1 Answers

You should use A.SubA.

As I know access to nested class via instance variable is not allowed from mypy point of view. Because derived classes from A could override nested class, and mypy cannot recognize this case, something like this:

class A:
    class SubA:
        pass

class C(A):
    class SubA:
        pass
    
def foo(a: A):
    class B(a.SubA):  # What SubA here ?
        pass

foo(C())

Update:

As for Flask-SQLAlchemy, the following workaround was suggested in this discussion:

from app import db
from sqlalchemy.ext.declarative import DeclarativeMeta

BaseModel: DeclarativeMeta = db.Model


class MyModel(BaseModel): ...

If you are using flask_sqlalchemy then you can use from flask_sqlalchemy.model import DefaultMeta instead of DeclarativeMeta.

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