检查 Python 列表中是否存在键
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Check if a key exists in a Python list
提问by fedorqui 'SO stop harming'
Suppose I have a list that can have either one or two elements:
假设我有一个可以包含一两个元素的列表：
mylist=["important", "comment"]
or
或者
mylist=["important"]
Then I want to have a variable to work as a flag depending on this 2nd value existing or not.
然后我想要一个变量作为标志，这取决于第二个值是否存在。
What's the best way to check if the 2nd element exists?
检查第二个元素是否存在的最佳方法是什么？
I already did it using len(mylist). If it is 2, it is fine. It works but I would prefer to know if the 2nd field is exactly "comment" or not.
我已经使用len(mylist). 如果是2，那就没问题了。它有效，但我更想知道第二个字段是否完全是“评论”。
I then came to this solution:
然后我来到了这个解决方案：
>>> try:
... c=a.index("comment")
... except ValueError:
... print "no such value"
...
>>> if c:
... print "yeah"
...
yeah
But looks too long. Do you think it can be improved? I am sure it can but cannot manage to find a proper way from the Python Data Structures Documentation.
但是看起来太长了。你认为它可以改进吗？我确信它可以但无法从Python Data Structures Documentation 中找到正确的方法。
采纳答案by arshajii
What about:
关于什么：
len(mylist) == 2 and mylist[1] == "comment"
For example:
例如：
>>> mylist = ["important", "comment"]
>>> c = len(mylist) == 2 and mylist[1] == "comment"
>>> c
True
>>>
>>> mylist = ["important"]
>>> c = len(mylist) == 2 and mylist[1] == "comment"
>>> c
False
回答by Rohit Jain
Use inoperator:
使用in运算符：
>>> mylist=["important", "comment"]
>>> "comment" in mylist
True
Ah! Missed the part where you said, you just want "comment"to be the 2nd element. For that you can use:
啊! 错过了你说的部分，你只想"comment"成为第二个元素。为此，您可以使用：
len(mylist) == 2 and mylist[1] == "comment"
回答by Martijn Pieters
You can use the inoperator:
您可以使用in运算符：
'comment' in mylist
or, if the positionis important, use a slice:
或者，如果位置很重要，请使用切片：
mylist[1:] == ['comment']
The latter works for lists that are size one, two or longer, and only is Trueif the list is length 2 andthe second element is equal to 'comment':
后者适用于大小为 1、2 或更长True的列表，并且仅适用于列表长度为 2且第二个元素等于'comment'：
>>> test = lambda L: L[1:] == ['comment']
>>> test(['important'])
False
>>> test(['important', 'comment'])
True
>>> test(['important', 'comment', 'bar'])
False
