我如何测试 python 列表中的空列表条目
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How do I test for null list entry in python list
提问by scopedial
I have a python list [1,2,,4,5,6]. What is the best method for testing for the missing, or null, list element? I am currently performing a if element != ''but I think there is a built-in test to perform such a thing, or am I wrong?
我有一个 python 列表[1,2,,4,5,6]。测试缺失或null列表元素的最佳方法是什么？我目前正在执行 aif element != ''但我认为有一个内置的测试来执行这样的事情，还是我错了？
采纳答案by Ewan
Pythonic ways for checking for Noneor nullare:
检查None或 的Pythonic 方法null是：
if element:
# This is not null
if not element:
# This is null
There is a very detailed answeron the difference between if not xand if x == None.
有一个非常详细的回答上的区别if not x和if x == None。
Edit 1:
编辑1：
Combining the comment and the other answers:
结合评论和其他答案：
False Values
错误值
Python treats the following as Falsesource:
Python 将以下内容视为False源代码：
NoneFalse- zero of any numeric type, eg:
0,0L,0.0,0j - any empty sequence, eg:
'',(),[] - any empty mapping, eg:
{} - instances of user-defined classes, if the class defines a
__nonzero__()or__len__()method, when that method returns the integerzeroor bool valueFalse
NoneFalse- 任何数字类型的零，例如：
0,0L,0.0,0j - 任何空序列，例如：
'',(),[] - 任何空映射，例如：
{} - 用户定义类的实例，如果类定义了一个
__nonzero__()或__len__()方法，当该方法返回整数zero或布尔值时False
True Values
真实值
All other values are considered to be True.
所有其他值都被视为True。
Your question:
你的问题：
If you are indeed checking if Noneis present in your list ([1,2,None,4,5,6]) then @Poke 's answer is right:
如果您确实要检查None列表中是否存在 ( [1,2,None,4,5,6])，那么@Poke 的答案是正确的：
>>> lst = [1, 2, None, 4, 5, 6]
>>> None in lst
True
If you are wanting to check that the element is onlyNonethen @esauro is correct in the comments:
如果您想检查该元素是否仅None在评论中@esauro 是正确的：
>>> lst = [1, 2, None, 4, 5, 6]
>>> for x in lst:
... if not x:
... print(x)
None
But if your lstcontains 0(lst = [0, 1, 2, 4, 5, 6]) then your output will be 0.
但是如果你lst包含0(lst = [0, 1, 2, 4, 5, 6]) 那么你的输出将是0.
The only way you could get round this would be to explicitly check if element is Noneor if element is not None.
您可以绕过此问题的唯一方法是明确检查if element is None或if element is not None。
回答by poke
If your list is [1, 2, None, 4, 5, 6]then you can just check None in lst, e.g.:
如果您的列表是，[1, 2, None, 4, 5, 6]那么您只需检查None in lst，例如：
>>> lst = [1, 2, None, 4, 5, 6]
>>> None in lst
True
Similarly, if you have some other value that represents a “missing” entry, you can just check whatever in lst.
同样，如果您有其他一些代表“缺失”条目的值，您只需检查whatever in lst.
回答by Emil Davtyan
Find all the indexes where there is a None:
找到所有有 的索引None：
[i for i, item in enumerate(mylist) if item is None]
Example output :
示例输出：
>>> mylist = [1, 2, None, 4, 5, 6, None, None]
>>> [i for i, item in enumerate(mylist) if item is None]
[2, 6, 7]
回答by Burhan Khalid
I have a python list [1,2,,4,5,6]. What is the best method for testing for the missing, or null, list element?
我有一个 python 列表 [1,2,,4,5,6]。测试缺失或空列表元素的最佳方法是什么？
If you just want to know if the list contains a falsey value(that's the best equivalent for missing or nullin Python), you can do this:
如果您只想知道列表是否包含falsey 值（这是Python 中缺失或 null的最佳等价物），您可以这样做：
>>> all(x for x in [1,2,3,4,5])
True
>>> all(x for x in [1,2,'',4,5])
False
>>> all(x for x in [1,2,None,4,5])
False
If you want to know if a specific value exists that matches some condition:
如果您想知道是否存在与某些条件匹配的特定值：
all(x%2 for x in [1,2,3,4,5,7])
This will be true if the list contains all even numbers.
如果列表包含所有偶数，则为真。
