为什么在 Python 中不允许单一类型约束? [英] Why are single type constraints disallowed in Python?

问题描述

假设你想约束一个类型变量来实现某个接口.你可以这样写:

Suppose you want to constrain a type variable to implement a certain interface. You might write something like so:

from typing import TypeVar, Callable

T = TypeVar('T', Callable)

class Foo(Generic[T]):
    ...

>> TypeError: A single constraint is not allowed

为什么 Python 对这种类型约束的使用不满意?PEP 484 和 Python 源代码 在这方面没有帮助.

Why is Python unhappy about this use of type constraints? PEP 484 and the Python source code are unhelpful in this regard.

注意:在我的特殊情况下，我对约束类型变量以实现抽象基类感兴趣，但原理是相同的.

Note: in my particular case I am interested in constraining a type variable to implement an abstract base class, but the principle is the same.

推荐答案

您正在寻找 bound:

T = TypeVar('T', bound=Callable)

来自文档:

类型变量可以使用 bound= 指定上限.这意味着替换(显式或隐式)类型变量的实际类型必须是边界类型的子类，请参阅 PEP 484.

a type variable may specify an upper bound using bound=<type>. This means that an actual type substituted (explicitly or implicitly) for the type variable must be a subclass of the boundary type, see PEP 484.

TypeVar(name, *args) 意味着类型必须是 args 之一，所以 T 的所有实例都只是如果允许 T = TypeVar('T', Callable)，则可替换为 Callable.

TypeVar(name, *args) means that the type has to be one of args, so all instances of T would just be replaceable by Callable if T = TypeVar('T', Callable) were allowed.

你应该可以在这里看到不同之处(虽然我没有真正尝试过，呵呵):

You should be able to see the difference here (though I didn’t actually try it, heh):

from typing import Generic, TypeVar, Callable

T = TypeVar('T', Callable, bool)

class Foo(Generic[T]):
    value: T

    def __init__(self, value: T) -> None:
        self.value = value

class Bar:
    baz = 5

    def __call__(self):
        pass

f = Foo(Bar())
print(f.value.baz)  # doesn’t typecheck because f.value is only a Callable

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