我有一个Python方法，它只能在泛型类的一个受限子集上工作。作为一个构造示例，我可能希望以下代码中的 Spam.floatify 只对 Spam[int] 的实例可用，即便我希望存在 Spam[str] 的实例：

class Spam[T]:
    def __init__(self, ham: T) -> None:
        self.ham: T = ham

    def floatify(self) -> float:
        return float(self.ham) # Errors, but shouldn't

Spam(0).floatify()
Spam("eggs")
Spam("eggs").floatify() # Should error, but doesn't

如何编写类型提示来检查这种行为？

解决方案

你其实可以用包含该参数的类的子集来对 self 参数进行类型提示！这将拒绝对不在该子集中的实例的调用，但不会拒绝创建这些实例。在你的示例中：

class Spam[T]:
    def __init__(self, ham: T) -> None:
        self.ham: T = ham

    def floatify(self: Spam[int]) -> float:
        return float(self.ham)

Spam(0).floatify()
Spam("eggs")
Spam("eggs").floatify() # Error

(在 mypy, Pyright, ty, and Pyrefly.)

Be aware of variance. Since T is invariant, this will only work for instances of Spam[int], not for parameterizations on subtypes of int like Spam[bool]. For those, you'll need to manually type-hint the instance at instantiation if possible:

Spam(False).floatify() # Error

bool_spam: Spam[int] = Spam(False)
bool_spam.floatify() # Ok

Also, if you're on pre-3.14, that forward reference needs to be wrapped in quotes (whether manually or with from __future__ import annotations).