如何为子类实例的列表添加类型注释，例如以串联两个列表？

如何解决如何为子类实例的列表添加类型注释，例如以串联两个列表？

我想遍历List[A]和List[Subclass of A]并进行相同的循环。我可以看到的最好方法是将两个列表连接起来。但是，mypy对此并不满意。

如何将两者连接起来并使mypy开心？

当前，我做# type: ignore[operator]。我想避免这种情况。

MVCE

# Core Library modules
from typing import Iterable

# Third party modules
from pydantic import BaseModel


class Animal(BaseModel):
    height: float
    weight: float


class Cat(Animal):
    lives: int = 7


cats = [Cat(height=1,weight=2,lives=7),Cat(height=3,lives=1)]
animals = [Animal(height=9,weight=9)]

combined: Iterable[Animal] = cats + animals

for animal in combined:
    print(animal)

给予

$ mypy untitled.py
untitled.py:20: error: Unsupported operand types for + ("List[Cat]" and "List[Animal]")
Found 1 error in 1 file (checked 1 source file)

解决方法

发生这种情况是因为list是invariant（提供了一个示例）。

我可以提供两种解决方案：

1. 将两个列表明确定义为List[Animal]才能成功进行串联：

cats: List[Animal] = [Cat(height=1,weight=2,lives=7),Cat(height=3,lives=1)]
animals: List[Animal] = [Animal(height=9,weight=9)]
combined: Iterable[Animal] = cats + animals

for animal in combined:
    print(animal)

1. 使用itertools.chain进行连续迭代：

cats = [Cat(height=1,lives=1)]
animals = [Animal(height=9,weight=9)]

for animal in itertools.chain(cats,animals):
    print(animal)

,

如果不打扰您，请将List更改为Sequence

from typing import Sequence

class Base: pass

class Derived(Base): pass

ds: Sequence[Derived] = [Derived()]
bs: Sequence[Base] = ds

您将获得的

$ mypy temp.py
Success: no issues found in 1 source file