Python泛型类联合类型分发疑问：Foo[T,A|B]与Foo[T,A]|Foo[T,B]

问题背景 #

我正在梳理Python泛型类型特化和联合类型的交互逻辑。针对泛型类Foo[T, U]，我原本想通过def op(self: Foo[T, type1 | type2 | ...]) -> rettype的方法签名，让Foo[T, type1]、Foo[T, type2]这类实例都能调用该方法，但实际无法生效，必须把联合类型拆成Foo[T, type1] | Foo[T, type2] | ...的形式才行。

项目动机 #

Foo类的op方法会返回其他Foo实例，新实例的生成依赖调用者的类型（类似链表节点）。为了支持链式调用（比如foo1.op1().op2().op3()），需要通过类型系统限制不兼容的类型调用对应的op方法。

核心疑问 #

Python中Foo[T, A | B | C]与Foo[T, A] | Foo[T, B] | Foo[T, C]的类型关系是什么？

测试代码与现象（Python 3.10 + mypy 1.1.1） #

# (Python 3.10, Linux, mypy 1.1.1)

from __future__ import annotations

from typing import Generic, TypeVar, cast

import numpy as np


T = TypeVar("T")
U = TypeVar("U")


class Foo(Generic[T, U]):
    def __init__(self, x: T, y: U):
        self.x = x
        self.y = y

    def op1(self: Foo[T, int | float]) -> float:
        return self.y * 2.5

    def op2(self: Foo[T, float] | Foo[T, int]) -> float:
        return self.y * 2.5

    def op3(self: Foo[T, int | float | str]) -> float:
        return cast(float, self.y) * 2.5
        
Foo(0, 1).op1()     # error: Invalid self argument "Foo[int, int]" to attribute function "op1" with type "Callable[[Foo[T, Union[int, float]]], float]"  [misc]
Foo(0, 1.0).op1()   # ok: 为什么这个能通过？而下面添加了str类型的op3调用全部失败？
Foo(0, "1").op1()   # error: Invalid self argument "Foo[int, str]" to attribute function "op1" with type "Callable[[Foo[T, Union[int, float]]], float]"  [misc]

Foo(0, 1).op2()     # ok
Foo(0, 1.0).op2()   # ok
Foo(0, "1").op2()   # error: Invalid self argument "Foo[int, str]" to attribute function "op2" with type "Callable[[Union[Foo[T, float], Foo[T, int]]], float]"  [misc]

Foo(0, 1).op3()     # error: Invalid self argument "Foo[int, int]" to attribute function "op3" with type "Callable[[Foo[T, Union[int, float, str]]], float]"  [misc]
Foo(0, 1.0).op3()   # error: Invalid self argument "Foo[int, float]" to attribute function "op3" with type "Callable[[Foo[T, Union[int, float, str]]], float]"  [misc]
Foo(0, "1").op3()   # error: Invalid self argument "Foo[int, str]" to attribute function "op3" with type "Callable[[Foo[T, Union[int, float, str]]], float]"  [misc]

具体困惑 #

我原本以为op1和op2是等价的，但实际op1只允许Foo[T, float]调用，Foo[T, int]调用会报错；而添加了str类型的op3则所有类型调用都报错，这是为什么？

内容的提问来源于stack exchange，提问作者TCM