如何为子类实例列表进行类型注释，例如连接两个列表？

Question

我想迭代

List[A]

和

List[Subclass of A]

并执行相同的循环。我认为做到这一点的最好方法是将两个列表连接起来。然而，mypy 对此并不高兴。

我怎样才能将两者连接起来并让我的朋友开心？

目前，我做

# type: ignore[operator]

。如果可能的话，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, weight=2, 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)

Answer 1

出现这种情况是因为

list

是 不变（提供了一个说明性示例）。

我可以提供两种解决方案：

1. 将两个列表显式定义为

   List[Animal]

   以成功串联：

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

for animal in combined:
    print(animal)

2. 使用itertools.chain进行连续迭代：

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

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

Answer 2

如果不介意的话，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

你会得到

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