Python 键入 - 约束列表只允许一种类型的子类

Question

I have 3 simple classes like:

class Animal(abc.ABC):
    ...

class Cat(Animal):
    ...

class Dog(Animal):
    ...

Then I have a function which is annotated as such:

def speak(animals: List[Animal]) -> List[str]:
   ...

My problem is that I want to constrain the List[Animal] to only include one type of animal, so:

speak([Dog(), Dog()]) # OK
speak([Cat(), Cat()]) # OK
speak([Cat(), Dog()]) # typing error

How would I annotate the speak function to allow for this? Is it even possible to do using typing or am I forced to check this at runtime?

I have tried to use the List[Animal] as above but that doesn't give me an error when calling speak like speak([Cat(), Dog()]) .

I have also tried messing around with generics like TypeVar('T', bound=Animal) but this still allows me to pass in a List of any combination of subclasses.

Answer 1

I would consider your issue to be not yet well defined. Once you start filling in a more concrete implementation of Animal , you're possibly going to arrive at a convincing solution.

Here, I'll reword your criteria for speak as it currently stands: You want it to accept a list of any individual subclass of Animal , but not Animal itself. Hopefully we can see why this doesn't make sense - there's nothing in your given code that suggests Animal can be distinguished from any subclass of Animal , at least from the point of view to what speak will do to the list of animals.

Let's provide some distinguishing features instead:

Python 3.10

import typing as t
from typing_extensions import LiteralString
from collections.abc import Sequence

Sound = t.TypeVar("Sound", bound=LiteralString)

class Animal(t.Generic[Sound]):
    def speak(self) -> Sound:
        ...

class Cat(Animal[t.Literal["meow"]]):
    ...

class Dog(Animal[t.Literal["bark"]]):
    ...

def speak(animals: Sequence[Animal[Sound]]) -> list[str]:
    return [animal.speak() for animal in animals]


>>> speak([Dog(), Dog()])  # OK
>>> speak([Cat(), Cat()])  # OK
>>>
>>> # mypy: Argument 1 to "speak" has incompatible type "List[object]"; expected "Sequence[Animal[<nothing>]]" [arg-type]
>>> # pyright: Argument of type "list[Cat | Dog]" cannot be assigned to parameter "animals" of type "Sequence[Animal[Sound@speak]]" in function "speak"
>>> # pyre: Incompatible parameter type [6]: In call `speak`, for 1st positional only parameter expected `Sequence[Animal[Variable[Sound (bound to typing_extensions.LiteralString)]]]` but got `List[Union[Cat, Dog]]`
>>> speak([Cat(), Dog()])

Note that although mypy doesn't complain about the signature squeak(animals: list[Animal[Sound]]) , this is technically not type-safe; you may decide to append Cat() to list[Dog] . This is why Sequence is used (it is non-mutable and covariant in its element types).