问题
x=[1,2,3]
x.extend('a')
Output:
x is [1,2,3,'a']
But when I do the following:
[1,2,3].extend('a')
Output:
None
Why does extend work on a list reference, but not on a list?
2nd Part:
I found this because I was trying to append a listB to a listA while trying to extend listC to listB.
listA.append([listB[15:18].extend(listC[3:12])])
Supposing lists cannot be directly appended / extending. What is the most popular work around form for resolving this issue?
回答1:
list.extend modifies the list in place and returns nothing, thus resulting in None. In the second case, it's a temporary list that is being extended which disappears immediately after that line, while in the first case it can be referenced via x.
to append a listB to a listA while trying to extend listC to listB.
Instead of using extend, you might want to try this:
listA.append(listB[15:18] + listC[3:12])
Or do it in multiple simple lines with extend if you want to actually modify listB or listC.
回答2:
extend will extend list it self. Return type of that method is None
If you want to union 2 list and add that list to another list then you have to use another way to add.
listB[15:18] = listC[3:12]
listA.extend(listB)
来源：https://stackoverflow.com/questions/22067425/python-extending-a-list-directly-results-in-none-why