Python 中的逻辑运算符 [英] Logical operators in Python

问题描述

在阅读 python 中的逻辑运算符时，我遇到了表达式:

 5 和 1

输出:1

 5 或 1

输出:5

谁能解释一下这是如何工作的?
我知道逻辑运算符的操作数是布尔

解决方案

即有据可查的:

x 或 y 如果 x 为假，则为 y，否则为 xx 和 y 如果 x 为假，则为 x，否则为 y

两者都短路(例如，如果 x 是 truthy，or 将不会评估 y).

文档还说明了什么被认为是falsy (False, 0, None, 空序列/映射, ...) - 其他的都被考虑真实.

几个例子:

7 和 'a' # ->'一种'[] 或无 # ->没有任何{'a': 1} 或 'abc'[5] # ->{'a': 1};没有从abc"[5] 引发的 IndexError假和 'abc'[5] # ->错误的;没有从abc"[5] 引发的 IndexError

注意最后两条语句是如何显示短路行为的:第二条语句(会引发 IndexError)没有执行.

您关于操作数是布尔值的说法有点没有实际意义.python 确实有布尔值(实际上只有 2 个:True 和 False；它们是 int 的子类型).但是python中的逻辑运算只是检查操作数是truthy还是falsy.bool 函数是未在操作数上调用.

<小时>

术语truthy 和falsy 似乎没有在官方python 文档中使用.但是教 Python 的书籍和这里的社区确实使用了这些术语.在 english.stackexchange.com 上有关于条款的讨论，在 维基百科.

While reading the Logical operators in python i came across with expressions :

  5 and 1 

output : 1

  5 or 1 

output : 5

Can anyone explain how is this working ?
I know that the operands of the logical operators are Boolean

解决方案

that is well documented:

x or y      if x is false, then y, else x 
x and y     if x is false, then x, else y 

both short-circuit (e.g. or will not evaluate y if x is truthy).

the documentation also states what is considered falsy (False, 0, None, empty sequences/mappings, ...) - everything else is considered truthy.

a few examples:

7 and 'a'             # -> 'a'
[] or None            # -> None
{'a': 1} or 'abc'[5]  # -> {'a': 1}; no IndexError raised from 'abc'[5]
False and 'abc'[5]    # -> False; no IndexError raised from 'abc'[5]

note how the last two show the short-circuit behavior: the second statement (that would raise an IndexError) is not executed.

your statement that the operands are boolean is a bit moot. python does have booleans (actually just 2 of them: True and False; they are subtypes of int). but logical operations in python just check if operands are truthy or falsy. the bool function is not called on the operands.

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the terms truthy and falsy seem not to be used in the official python documentation. but books teaching python and the community here do use these terms. there is a discussion about the terms on english.stackexchange.com and also a mention on wikipedia.

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