How to type python class factory with linter?

Question

I have this code

from abc import ABC
from typing import Type


class AbstractFoo(ABC):
    """Abstract foo."""
    foo_attr: int


def foo_factory(foo_attr_val: int) -> Type[AbstractFoo]:

    class Foo(AbstractFoo):
        foo_attr = foo_attr_val

    return Foo


FooA = foo_factory(5)


class FooB(FooA):
    """Public concrete foo."""

print(FooB.foo_attr)

This executes just fine, but then I run mypy with mypy foo.py , I get an error that looks like

foo.py:21: error: Variable "foo.FooA" is not valid as a type
foo.py:21: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
foo.py:21: error: Invalid base class "FooA"
Found 2 errors in 1 file (checked 1 source file)

I don't understand what is wrong here and why this type is invalid. What's the way to fix this?

I'm currently on Python 3.9 with mypy version 0.971.

Answer 1

Your problem is that mypy does not support dynamic base classes (doesn't matter if they are well-defined). See this mypy issue for more workarounds and details.

If you're happy enough to have a factory producing class without major modifications, you can just pretend that return type is that class ( try me online! ):

from abc import ABC
from typing import Type


class AbstractFoo(ABC):
    """Abstract foo."""
    foo_attr: int


def foo_factory(foo_attr_val: int) -> Type[AbstractFoo]:

    class Foo(AbstractFoo):
        foo_attr = foo_attr_val

    return Foo


if TYPE_CHECKING:
    FooA = AbstractFoo
else:
    FooA = foo_factory(5)


class FooB(FooA):
    """Public concrete foo."""

print(FooB.foo_attr)

TYPE_CHECKING is a special constant that is False at runtime and True for type checkers. Here you make type checker think that your FooA is just a type alias (and on py3.10+ or with typing_extensions you can even make it explicit with TypeAlias type annotation).

Answer 2

Based on the comments, it appears as if this is a mypy issue. I did find that with Python 3.10 on MAC, the code still required some changes to run correctly:

class AbstractFoo:
    foo_attr: int

def foo_factory(foo_attr: int) -> type[AbstractFoo]:

    class Foo(AbstractFoo):
        AbstractFoo.foo_attr=foo_attr

    return Foo


FooA = foo_factory(5)

class FooB(FooA):
  pass

I made two changes:

1. Type -> type
2. foo_attr --> AbstractFoo.foo_attr