April 2023
Intermediate to advanced
769 pages
25h 16m
Chinese
Content preview from 流畅的Python（第2版）
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类型提示进阶
｜
409
➊
这个示例的目的是在函数签名中使用
BookDict
。
➋
一开始为空的容器通常需要注解，否则
Mypy
无法推导出元素的类型。
5
➌
Mypy
能理解
isinstance
所做的检查，并会在这个代码块中把
value
看作一个
list
。
➍
最初，守护这个代码块的
if
条件是
key == 'authors'
，但是
Mypy
报错：
"object" has
no attribute "__iter__"
。这是因为，经
Mypy
推 导，
book.items()
返回的
value
的
类型为
object
，而
object
不支持这个生成器表达式所需的
__iter__
方法。后来换成
isinstance
就好了，因为
Mypy
知道，在这个代码块中，
value
的值是一个
list
。
示例
15-9
定义了一个解析
JSON
字符串并返回
BookDict
的函数。
示例
15-9
books_any.py
：
from_json
函数
def from_json(data: str) -> BookDict:
whatever = json.loads(data)
➊
return whatever
➋
➊
json.loads()
的返回值类型为
Any
。
6
➋
可以返回
whatever
（
Any
类型），因为
Any
与任何类型都
相容
，包括声明的返回值类型
BookDict
。
要特别注意示例
15-9
的第
2
个标
号处。
Mypy
不会标记那行代码有问题，但是在运行时，
whatever
的值或许不满足
BookDict
结构——甚至可能根本就不是字典。
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and much more.
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
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.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
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.