不明白为什么变量为整数时条件求值为True [英] Do not understand why if condition evaluates to True when variable is integer

问题描述

我正在处理表现以下行为的代码.假设我有一个变量d，并将其分配给一个整数9

I am working with code that exhibits the following behavior. Suppose I have a variable d and I assign it to an integer 9

d = 9

以下语句为什么起作用?

why do the following statements work?

In [95]: if d:
   ....:     print d
   ....: else:
   ....:     print 'Did not print d!'
   ....:     
9

In [96]: 

当d本身不是布尔值且未通过以下测试时:

when d itself is not a Boolean and does not pass the following tests:

In [96]: d is True
Out[96]: False

In [97]: d is False
Out[97]: False

如果有人可以向我解释一下并指出我的任何误解，我将不胜感激.非常感谢.

I will appreciate it if someone could explain this to me and point out any misconception that I have. Thank you very much.

推荐答案

请记住，is测试身份.您的d is True和d is False语句返回False，因为d是9，而不是True或False.但是，d的真实值为True-尝试运行bool(d)以获得其布尔值(提示:它将返回True).

Remember, is tests for identity. Your d is True and d is False statements are returning False because d is 9, not True or False. However, the truthy value of d is True - try running bool(d) to get its Boolean value (hint: it'll return True).

如其他地方所述，False，None，0和空字符串，列表，字典，集合，元组等都被认为是虚假的.根据对象的__nonzero__和/或__len__属性(如果已设置)的内容，它们也可能是虚假的.

As stated elsewhere, False, None, 0, and empty strings, lists, dicts, sets, tuples, etc. are all considered falsy. Objects can also be falsy depending on the contents of their __nonzero__ and/or __len__ attributes, if set.

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