Python 空列表是否等于 None ？
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Empty list is equal to None or not?
提问by user1559873
Possible Duplicate:
Why does “[] == False” evaluate to False when “if not []” succeeds?
I am new to python as per ternary operator of python
根据python的三元运算符，我是python的新手
>>> 'true' if True else 'false' true
true
i am expecting for below code output as [] because [] not equal to None
我期待以下代码输出为 [] 因为 [] 不等于 None
>>> a=[]
>>> a==None
False
>>> a if a else None
None
pleas correct if i am wrong
如果我错了，请纠正
Thanks hema
谢谢盒马
采纳答案by jdotjdot
The empty list, [], is notequal to None.
空单，[]是不是等于None。
However, it canevaluate to False--that is to say, its "truthiness" value is False. (See the sources in the comments left on the OP.)
但是，它可以求值为False——也就是说，它的“真实性”值为False。（请参阅 OP 上留下的评论中的来源。）
Because of this,
因为这，
>>> [] == False
False
>>> if []:
... print "true!"
... else:
... print "false!"
false!
回答by ank
Noneis the sole instance of the NoneTypeand is usually used to signify absence of value. What happens in your example is that the empty list, taken in boolean context, evaluates to False, the condition fails, so the else branch gets executed. The interpreter does something along the lines of:
None是 的唯一实例，NoneType通常用于表示没有值。在您的示例中发生的情况是，在布尔上下文中采用的空列表评估为False，条件失败，因此执行 else 分支。解释器做一些事情：
>>> a if a else None
[] if [] else None
[] if False else None
None
Here is another useful discussion regarding None: not None test in Python
这是另一个关于None：not None test in Python 的有用讨论
