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。我能看到的最好的方法是将这两个列表连接起来。然而，mypy对此并不高兴。

我怎样才能把两者连接起来，让mypy开心呢？

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)

给出

$ 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是不变的(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]，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

如果不困扰您，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

用它你会得到

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