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HomeBackend DevelopmentPython TutorialPython and、or以及and-or语法总结

Python and、or以及and-or语法总结

May 26, 2017 am 11:56 AM
andorpythongrammar

一、and:

Python 中,and 和 or 执行布尔逻辑演算,如你所期待的一样,但是它们并不返回布尔值;而是,返回它们实际进行比较的值之一。

代码如下:

>>> 'a' and 'b' 'b' >>> '' and 'b' '' >>> 'a' and 'b' and 'c' 'c'

在布尔上下文中从左到右演算表达式的值,如果布尔上下文中的所有值都为真,那么 and 返回最后一个值。

如果布尔上下文中的某个值为假,则 and 返回第一个假值

二、or:

代码如下:

>>> 'a' or 'b' 'a' >>> '' or 'b' 'b' >>> '' or [] or {}
{} >>> 0 or 'a' or 'c' 'a'
[code]

使用 or 时,在布尔上下文中从左到右演算值,就像 and 一样。如果有一个值为真,or 立刻返回该值

如果所有的值都为假,or 返回最后一个假值

注意 or 在布尔上下文中会一直进行表达式演算直到找到第一个真值,然后就会忽略剩余的比较值

三、and-or:

and-or 结合了前面的两种语法,推理即可。

[

code]>>> a='first' >>> b='second' >>> 1 and a or b 'first' >>> (1 and a) or b 'first' >>> 0 and a or b 'second' >>> (0 and a) or b 'second' >>>

这个语法看起来类似于 C 语言中的 bool ? a : b 表达式。整个表达式从左到右进行演算,所以先进行 and 表达式的演算。 1 and 'first' 演算值为 'first',然后 'first' or 'second' 的演算值为 'first'。

0 and 'first' 演算值为 False,然后 0 or 'second' 演算值为 'second'。

and-or主要是用来模仿 三目运算符 bool?a:b的,即当表达式bool为真,则取a否则取b。

and-or 技巧,bool and a or b 表达式,当 a 在布尔上下文中的值为假时,不会像 C 语言表达式 bool ? a : b 那样工作。

四、安全使用and-or

代码如下:

>>> a="" >>> b="second" >>> (1 and [a] or [b])
[''] >>> (1 and [a] or [b])[0] '' >>>


由于 [a] 是一个非空列表,所以它决不会为假。即使 a 是 0 或者 '' 或者其它假值,列表 [a] 也为真,因为它有一个元素。

一个负责的程序员应该将 and-or 技巧封装成一个函数

代码如下:

def choose(bool,a,b): return (bool and [a] or [b])[0] print choose(1,'','second') #''

【相关推荐】

1. 解析python中and与or用法

2. 详细介绍Python中and和or实际用法

3. 分享python中and / or 的运算逻辑实例教程

4. 总结Python的逻辑运算符and

5. Python:逻辑判断与运算符实例

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