April 2023
Intermediate to advanced
769 pages
25h 16m
Chinese
Content preview from 流畅的Python（第2版）
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424
｜
第
15
章
不可变容器可以是协变的。例如，文档中使用约定的命名方式
T_co
指明
typing.FrozenSet
有一个可协变的类型变量。
class FrozenSet(frozenset, AbstractSet[T_co]):
使用
:>
表示参数化类型，如下所示。
float :> int
frozenset[float] :> frozenset[int]
迭代器也可以是协变的。迭代器不是
frozenset
这种只读的容器，而是只产生输出。只要
预期产出浮点数的
abc.Iterator[float]
，就可以放心使用产出整数的
abc.Iterator[int]
。
基于同样的原因，
Callable
类型的返回值类型也可以是协变的。
3.
逆变类型
对于
A :> B
，当满足
K[A] <: K[B]
时，泛型
K
是可逆变的。
可逆变的泛型可以逆转具体类型参数的子类型关系。
TrashCan
类就是一例。
Refuse :> Biodegradable
TrashCan[Refuse] <: TrashCan[Biodegradable]
可逆变的容器通常是只写的数据结构（也叫“接收器”，
sink
）。标准库中没有这样的容器，
不过一些类型有可逆变的类型参数。
Callable[[ParamType, ...], ReturnType]
中的参数类型是可逆变的，不过
ReturnType
是可协
变的（详见“
Callable
类型的型变”一节）
。另外，
Generator
、
Coroutine
和
AsyncGenerator
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