enumerating concrete subclasses of an abstract base class causes false positive "Cannot instantiate abstract class X with abstract attribute Y"

[poppadum]

I have a number of concrete classes which all derive from the same abstract base class. Creating a list of instances using __subclasses__() in a list comprehension, mypy complains that I'm trying to instantiate the abstract class.

Simple example:
https://mypy-play.net/?mypy=latest&python=3.11&gist=f03c51fac1173ae0bb751b00e614de68

from abc import ABC, abstractmethod
from typing import List

class Animal(ABC):
    @abstractmethod
    def talk(self) -> str:
        pass

class Dog(Animal):
    def talk(self) -> str:
        return "woof"

class Cat(Animal):
    def talk(self) -> str:
        return "miaow"

# enumerate using __subclasses__()
animals: List[Animal] = [
    subclass() for subclass in Animal.__subclasses__()
]
print(animals)

# hard-coding subclass instances is fine
animals2: List[Animal] = [
     Dog(), Cat()
]

Expected Behavior
As I'm not actually instantiating the abstract class, the mypy error looks like a false positive.

Your Environment
python 3.10.12
mypy v1.6.0

Possibly related issues
#15554
#14106
#3115

[darthwalsh]

@poppadum I think the mypy hint is right here, because __subclasses__() returns list[type[Animal]] and any of these could by abstract types.

Here's a different example:

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def talk(self) -> str:
        pass

class Dog(Animal):
    def talk(self) -> str:
        return "woof"

class Reptile(Animal):
    def talk(self) -> str:
        return "hiss"
    
    @abstractmethod
    def egg_type(self) -> str:
        pass

animals: list[Animal] = [
    subclass() for subclass in Animal.__subclasses__()
]
print(animals)

And creating Reptile() will fail:

$ python3 a.py
Traceback (most recent call last):
  File "/Users/walshca/a.py", line 20, in <module>
    animals: list[Animal] = [
                            ^
  File "/Users/walshca/a.py", line 21, in <listcomp>
    subclass() for subclass in Animal.__subclasses__()
    ^^^^^^^^^^
TypeError: Can't instantiate abstract class Reptile with abstract method egg_type
NativeCommandExitException: Program "python3" ended with non-zero exit code: 1.

$ mypy a.py
a.py:21: error: Cannot instantiate abstract class "Animal" with abstract attribute "talk"  [abstract]
Found 1 error in 1 file (checked 1 source file)
NativeCommandExitException: Program "mypy" ended with non-zero exit code: 1.

---

Instead, "enumerating concrete subclasses" would need to return something like typing.Concrete[Animal] (which doesn't exist), filtering __subclasses__() somehow (and probably recurse through subclasses of subclasses)