“如果不是"，则为“否".python中的条件语句 [英] "If not" condition statement in python

问题描述

if not start: 
   new.next = None 
   return new

如果不是"是什么意思?该代码何时执行?

what does "if not" mean? when will this code execute?

和说的一样吗如果开始==无:那要做什么?

is it the same thing as saying if start == None: then do something?

推荐答案

如果是语句. not start 是表达式，其中 not 是

if is the statement. not start is the expression, with not being a boolean operator.

not 返回 True .Python认为所有对象均为true，除非它们是数字零，空容器或 None 对象或布尔值 False .如果 start 是一个真值，则 not 返回 False .请参见 真值测试部分在文档中.

not returns True if the operand (start here) is considered false. Python considers all objects to be true, unless they are numeric zero, or an empty container, or the None object or the boolean False value. not returns False if start is a true value. See the Truth Value Testing section in the documentation.

因此，如果 start 为 None ，则实际上 not start 将为true. start 也可以为 0 ，或者为空列表，字符串，元组字典或集合.许多自定义类型还可以指定它们等于数字0或应视为空:

So if start is None, then indeed not start will be true. start can also be 0, or an empty list, string, tuple dictionary, or set. Many custom types can also specify they are equal to numeric 0 or should be considered empty:

>>> not None
True
>>> not ''
True
>>> not {}
True
>>> not []
True
>>> not 0
True

注意:因为 None 是单例(在Python进程中只有该对象的一个​​副本)，所以应始终使用 is 或不是.如果您严格地要测试tat start 为 None ，请使用:

Note: because None is a singleton (there is only ever one copy of that object in a Python process), you should always test for it using is or is not. If you strictly want to test tat start is None, then use:

if start is None:

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