Transpose list of lists
map(list, zip(*l))--> [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
For python 3.x users can use
list(map(list, zip(*l))) # short circuits at shortest nested list if table is jaggedlist(map(list, itertools.zip_longest(*l, fillvalue=None))) # discards no data if jagged and fills short nested lists with None
There are two things we need to know to understand what's going on:
- The signature of zip:
zipexpects an arbitrary number of arguments each of which must be iterable. E.g.
zip([1, 2], [3, 4], [5, 6]).
- Unpacked argument lists: Given a sequence of arguments
fsuch that each element in
argsis a separate positional argument of
itertools.zip_longestdoes not discard any data if the number of elements of the nested lists are not the same (homogenous), and instead fills in the shorter nested lists then zips them up.
Coming back to the input from the question
l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]],
zip(*l) would be equivalent to
zip([1, 2, 3], [4, 5, 6], [7, 8, 9]). The rest is just making sure the result is a list of lists instead of a list of tuples.