flatten list of lists of lists to a list of lists

Question

I\'ve already searched SO for how to flatten a list of lists (i.e. here:Making a flat list out of list of lists in Python) but none of the solutions I find addresses flatten

Answer 1

For this particular case,

In [1]: [sum(x,[]) for x in my_list]
Out[1]: [[1, 2, 3, 4, 5], [9, 8, 9, 10, 3, 4, 6], [1]]

is the shortest and the fastest method:

In [7]: %timeit [sum(x,[]) for x in my_list]
100000 loops, best of 3: 5.93 µs per loop

Answer 2

This is an inside-out version of fl00r's recursive answer which coincides more with what OP was after:

def flatten(lists,n):
    if n == 1:
        return [x for xs in lists for x in xs]   
    else:
        return [flatten(xs,n-1) for xs in lists]


>>> flatten(my_list,1)
[[1, 2, 3], [4, 5], [9], [8, 9, 10], [3, 4, 6], [1]]
>>> flatten(my_list,2)
[[1, 2, 3, 4, 5], [9, 8, 9, 10, 3, 4, 6], [1]]

Answer 3

If we apply the logic from this answer, should not it be just:

In [2]: [[item for subsublist in sublist for item in subsublist] for sublist in my_list]
Out[2]: [[1, 2, 3, 4, 5], [9, 8, 9, 10, 3, 4, 6], [1]]

And, similarly via itertools.chain():

In [3]: [list(itertools.chain(*sublist)) for sublist in my_list]
Out[3]: [[1, 2, 3, 4, 5], [9, 8, 9, 10, 3, 4, 6], [1]]

Answer 4

You could use this recursive subroutine

def flatten(lst, n):
  if n == 0:
    return lst

  return flatten([j for i in lst for j in i], n - 1)

mylist = [ [ [1,2,3],[4,5] ], [ [9],[8,9,10],[3,4,6] ], [ [1] ] ]
flatten(mylist, 1)
#=> [[1, 2, 3], [4, 5], [9], [8, 9, 10], [3, 4, 6], [1]]
flatten(mylist, 2)
#=> [1, 2, 3, 4, 5, 9, 8, 9, 10, 3, 4, 6, 1]