问题
practising some python, which is a pretty easy language to grab up.
I have
>>> L = [1,2,3,4]
>>> L[1:1] = [1,2,3]
>>> L
[1, 1, 2, 3, 2, 3, 4]
so on line two actually L[1:1] is empty list, but how can python understand that insert the [1,2,3] list to starting from 1. I guess there is some internals which is not transparent to us, here apparently, I guess L[1:1] returns a reference to index 1 even if that returns an empty list...
Best wishes, Umut
回答1:
L[1:1] means the slice of the list L starting at index 1 (the second element), up to but not including index 1. So it is an empty list. On the right-hand side of an assignment, it is simply an anonymous empty list. But on the left-hand side, the assignment knows where the slice has been made, and can splice in the new list value into the proper place.
回答2:
Slicing behaves differently depending on whether it's on the left- or right-hand side of an expression. When it's on the left side, it doesn't return a list - instead, it behaves as a slice object, which knows more about slices and has assignment specifically overridden to operate as insertion.
回答3:
The official Python Tutorial explains it best, in my opinion. The end of Chapter 3.1.2 has the following diagram:
+---+---+---+---+---+
| H | e | l | p | A |
+---+---+---+---+---+
0 1 2 3 4 5
What this illustrates is that you can think of the indices as pointing BETWEEN the elements. So in this illustration, if specifying a slice [1:1], you are actually referring to the space between H and e, but not including them.
If you wanted to overwrite H and e, you would specify the slice [0:2].
来源：https://stackoverflow.com/questions/4574487/slicing-insert-question-l11