实例属性的可区分联合

Question

假设我有以下课程

class MyClass:
    def __init__(self, ...):
        self.attr1: Union[Attr11, Attr12]
        self.attr2: Literal["val1", "val2", "val3"]
        self.attr3: Attr3
        ...
        self.attrn: Attrn

我对以下类型提示感兴趣：

if self.attr2 == "val1":
    # self.attr1 should be Attr11 here
else:
    # self.attr1 should be Attr12 here

我该怎么做？

Answer 1

您可以通过即将推出的实验性

TypeForm

来实现这一目标。

注意：目前（2025 年 1 月）仅由

pyright

支持并启用了实验性功能，例如您需要在

pyproject.toml

文件中添加以下行。

[tool.pyright]
enableExperimentalFeatures = true

---

from typing import Union, Literal, TypeGuard, Any, TypeVar
from typing_extensions import TypeForm, TypeIs

T = TypeVar("T")

def _narrow_type(attr: Any, condition: bool, to_type: TypeForm[T]) -> TypeIs[T]:
    return condition


class MyClass:
    def __init__(self, *args):
        self.attr1: Union[int, str]
        self.attr2: Literal["val1", "val2", "val3"]

        if _narrow_type(self.attr1, self.attr2 == "varl", int):
            reveal_type(self.attr1)  # Type of "self.attr1" is "int"
        else:
            # Note: using TypeGuard instead of TypeIs will not narrow the type, i.e. it stays int | str
            reveal_type(self.attr1)  # Type of "self.attr1" is "str"