I was wondering how to reverse a list of lists in python. For example,
Original: list = [[1,2,3],[4,5,6],[7,8,9]]
Output: new_list = [[7,8,9],[4,5,6],[1,2,3]]
Right now, I am trying this:
new_list = reversed(list)
However, this doesn't seem to work.
In [24]: L = [[1,2,3],[4,5,6],[7,8,9]]
In [25]: L[::-1]
Out[25]: [[7, 8, 9], [4, 5, 6], [1, 2, 3]]
reversed is lazily returning the reversed elements of the outer list; you need to iterate the value it returns, or wrap in a list constructor to convert it to a new list. Alternatively, you can reverse lists in place by calling .reverse(). The L[::-1] syntax is the canonical "return shallow copy in reverse order" approach though.
[::-1]?
start and end arguments are omitted, it signifies using the whole set of indices AND the step argument of -1 instructs the interpreter to begin at the end of the set?start should be the last index and stop should be just before the first index. Thus your step of -1 goes from the last to the first index