Type annotations for dynamically created subclasses

Question

I want to dynamically create child classes at startup with the type() function. Everything works as expected. I know this isn't great, but I am bound to a framework and have to do it this way. The other option is to generate source code...

Functional equivalent to my code:

class BaseEntity:
    id: str


def make_new_entity(name: str, attrs: dict) -> type:
    return type('RuntimeEntity', (BaseEntity,), attrs)


RuntimeEntity: ??? = make_new_entity('RuntimeEntity', {'id': 'entity.RuntimeEntity'})

Is there a way to provide a bound to the returned type? Basically the equivalent of

E = TypeVar('E', bound='BaseEntity')

I have also looked at the types module.

Thanks for any help!

chepner · Accepted Answer · 2021-07-22 16:04:47Z

3

typing.Type lets you specify that the value should be an actual type, rather than an instance of that type.

from typing import Type

class BaseEntity:
    id: str


def make_new_entity(name: str, attrs: dict) -> Type[BaseEntity]:
    return type('RuntimeEntity', (BaseEntity,), attrs)


RuntimeEntity: Type[BaseEntity] = make_new_entity('RuntimeEntity', {'id':     'entity.RuntimeEntity'})

A value of type Type[BaseEntity] can be BaseEntity itself, or any class that inherits from BaseEntity. (The term to describe this is covariance. If Type were invariant, Type[BaseEntity] would accept BaseEntity itself only.)

answered Jul 22, 2021 at 16:04

chepner

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3 Comments

bobrossofcoding Over a year ago

That's great, thank you! I tried that, but got confused as PyCharm says "Expected type 'Type[BaseEntity]', got 'type' instead". IntelliSense works on the instance though. Probably a bug or impossible to lint.

bobrossofcoding Over a year ago

It's even worse in the dict I'm putting it in: "Expected type Dict[str, Type[BaseEntity]], got Dict[str, type]" instead

chepner Over a year ago

I don't get any error with mypy on the code shown.

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