April 2023
Intermediate to advanced
769 pages
25h 16m
Chinese
Content preview from 流畅的Python（第2版）
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532
｜
第
18
章
LBYL
三思而后行（
Look
before you leap
）。这种编程风格在调用函数或查找属性或键之前
显式测试前提条件。与
EAFP
风格相比
，这种风格的特点是代码中有很多
if
语句。
在多线程环境中，
LBYL
风格可能会在
“检查”和“行事”的空隙引入条件竞争。
例如，对
if key in mapping: return mapping[key]
这段代码来说，如果在测试之
后——但在查找之前——另一个线程从映射中删除了那个键，那么这段代码就会失
败。这个问题可以使用锁或者
EAFP
风格解决。
如果选择使用
EAFP
风格，那就要更深入地了解
else
子句，并在
try
/
except
语句中合理使用。
讨论
match
语句时，有些人（包括我）认为
match
语句也应该有一个
else
子
句。最后的决定是不需要这个子句，因为
case _:
具有同样的作用。
13
下面对本章做个小结。
18.5
本章小结
本章首先讨论了上下文管理器和
with
语句的作用。很快我们就知道，除了自动关闭打开
的文件之外，
with
语句还有很多用途。我们自己动手实现了一个上下文管理器——定义了
有
__enter__
/
__exit__
方法的
LookingGlass
类，说明了如何在
__exit__
方法中处理异常。
Raymond Hettinger
在
PyCon US 2013
上所做的主题演讲传达了一个重要的观点
：
with
不仅
能管理资源，还可用于去掉常规的设置和清理代码，或者在另一个过程前后执行的操作。
14
随后，我们分析了标准库中
contextlib ...
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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.