为什么 List 不能包含多种类型? [英] Why can't List contain multiple types?

问题描述

您可以在元组或列表中混合类型.为什么不能在输入提示中指定?

<预><代码>>>>从输入导入元组，列表>>>t = ('a', 1)>>>l = ['a', 1]>>>t2: 元组[str, int] = ('a', 1)>>>l2:列表[str, int] = ['a', 1]类型错误:用于输入的参数太多.列表；实际 2，预期 1

解决方案

在类型理论中，列表是包含一种类型值的同构结构.因此，List 只接受一种类型，并且该列表的每个元素都必须具有该类型.

然而，类型理论还提供和类型，您可以将其视为从一组固定类型中选择的一个值的包装器.typing.Union 支持和类型.要指定列表是 int 和 str 值的混合，请使用

List[Union[str, int]]

作为类型提示.

相比之下，元组是产品类型的一个例子，一种由一组固定类型组成的类型，其值是一组值，来自产品类型中的每个类型.Tuple[int,int,int]、Tuple[str,int] 和 Tuple[int,str] 都是不同的类型，区分两者由产品中的类型的数量和它们出现的顺序决定.

You can mix types inside tuples or lists. Why can't you specify that in typing hints?

>>> from typing import Tuple, List
>>> t = ('a', 1)
>>> l = ['a', 1]

>>> t2: Tuple[str, int] = ('a', 1)
>>> l2: List[str, int] = ['a', 1]

TypeError: Too many parameters for typing.List; actual 2, expected 1

解决方案

In type theory, a list is a homogenous structure containing values of one type. As such, List only takes a single type, and every element of that list has to have that type.

However, type theory also provides sum types, which you can think of as a wrapper around exactly one value selected from some fixed set of types. A sum type is supported by typing.Union. To specify that a list is a mix of int and str values, use

List[Union[str, int]]

as the type hint.

By contrast, a tuple is an example of a product type, a type consisting of a fixed set of types, and whose values are a collection of values, one from each type in the product type. Tuple[int,int,int], Tuple[str,int] and Tuple[int,str] are all distinct types, distinguished both by the number of types in the product and the order in which they appear.

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