April 2023
Intermediate to advanced
769 pages
25h 16m
Chinese
Content preview from 流畅的Python（第2版）
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502
｜
第
17
章
my_coro : Generator[YieldType, SendType, ReturnType]
从形式参数中的类型变量可以看出，
YieldType
和
ReturnType
可以协变，
SendType
可以逆
变。为了理解其中的原因，可以把
YieldType
和
ReturnType
看作“输出”类型，描述的都
是从协程对象（即用作协程的生成器对象）中获取的数据。
这两个类型参数可以协变的原因不难理解，因为预取产出浮点数的协程自然可以使用产出
整数的协程。这是
YieldType
参数可以协变的原因，同样也适用于可以协变的
ReturnType
参数。
根据
15.7.4
节的
“协变类型”提出的表示法，可协变的第
1
个和第
3
个参数可以使用
:>
符号表达，指向相同的方向。
float :> int
Generator[float, Any, float] :> Generator[int, Any, int]
YieldType
和
ReturnType
体现了
15.7.4
节的“型变经验法则”给出的第一条法则。
•
如果一个形式类型参数定义的是从对象中获取的数据类型，那么该形式类型参数
可能是协变的。
另一方面，
SendType
是“输入”参数，是协程对象
.send(value)
方法的
value
参数的类
型。如果客户代码需要向协程发送浮点数，那就不能使用
SendType
为
int
的协程，因为
float
不是
int
的子类型。也就是说，
float
与
int
不相容。但是，可以使用
SendType
为
complex
的协程，因为 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.