约束的TypeVar和Union之间有什么区别? [英] What's the difference between a constrained TypeVar and a Union?

问题描述

如果我想要一个可以表示多种可能类型的类型，那么Union似乎就是我的表达方式:

If I want to have a type that can represent multiple possible types, Unions seem to be how I represent that:

U = Union[int, str] 

U可以是int或str.

我注意到，尽管TypeVar允许可选的var-arg参数，但它们似乎也做同样的事情:

I noticed though that TypeVars allow for optional var-arg arguments that also seem to do the same thing:

T = TypeVar("T", int, str)

T和U似乎都只能采用str和int类型.

Both T and U seem to only be allowed to take on the types str and int.

这两种方式之间有什么区别，什么时候应该优先使用?

What are the differences between these two ways, and when should each be preferred?

推荐答案

T的类型必须在给定的

T's type must be consistent across multiple uses within a given scope, while U's does not.

使用Union类型作为函数参数时，参数以及返回类型都可以不同:

With a Union type used as function parameters, the arguments as well as the return type can all be different:

U = Union[int, str]

def union_f(arg1: U, arg2: U) -> U:
    return arg1

x = union_f(1, "b")  # No error due to different types
x = union_f(1, 2)  # Also no error
x = union_f("a", 2)  # Also no error
x # And it can't tell in any of the cases if 'x' is an int or string

将其与参数类型必须匹配的TypeVar进行比较:

Compare that to a similar case with a TypeVar where the argument types must match:

T = TypeVar("T", int, str)

def typevar_f(arg1: T, arg2: T) -> T:
    return arg1

y = typevar_f(1, "b")  # "Expected type 'int' (matched generic type 'T'), got 'str' instead
y = typevar_f("a", 2)  # "Expected type 'str' (matched generic type 'T'), got 'int' instead

y = typevar_f("a", "b")  # No error
y  # It knows that 'y' is a string

y = typevar_f(1, 2)  # No error
y  # It knows that 'y' is an int

因此，如果允许多种类型，请使用TypeVar，但是在单个作用域内T的不同用法必须彼此匹配.如果允许使用多种类型的U，请使用Union，但是在给定范围内的不同用法不需要​​相互匹配.

So, use a TypeVar if multiple types are allowed, but different usages of T within a single scope must match each other. Use a Union if multiple types of U are allowed, but different usages within a given scope don't need to match each other.

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