Transpose list of lists

Question

Let's take:

l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] 

The result I'm looking for is

r = [[1, 4, 7], [2, 5, 8], [3, 6, 9]] 

and not

r = [(1, 4, 7), (2, 5, 8), (3, 6, 9)] 

Much appreciated

Answer 1

Python 3:

# short circuits at shortest nested list if table is jagged: list(map(list, zip(*l)))  # discards no data if jagged and fills short nested lists with None list(map(list, itertools.zip_longest(*l, fillvalue=None))) 

Python 2:

map(list, zip(*l)) 

[[1, 4, 7], [2, 5, 8], [3, 6, 9]] 

Explanation:

There are two things we need to know to understand what's going on:

1. The signature of zip: zip(*iterables) This means zip expects an arbitrary number of arguments each of which must be iterable. E.g. zip([1, 2], [3, 4], [5, 6]).
2. Unpacked argument lists: Given a sequence of arguments args, f(*args) will call f such that each element in args is a separate positional argument of f.
3. itertools.zip_longest does 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.

Answer 2

One way to do it is with NumPy transpose. For a list, a:

>>> import numpy as np >>> np.array(a).T.tolist() [[1, 4, 7], [2, 5, 8], [3, 6, 9]] 

Or another one without zip:

>>> map(list,map(None,*a)) [[1, 4, 7], [2, 5, 8], [3, 6, 9]]