Making a flat list out of list of lists in Python -

i wonder whether there shortcut make simple list out of list of lists in python.

i can in loop, maybe there cool "one-liner"? tried reduce, error.

code

l = [[1, 2, 3], [4, 5, 6], [7], [8, 9]] reduce(lambda x, y: x.extend(y), l) 

error message

traceback (most recent call last):   file "<stdin>", line 1, in <module>   file "<stdin>", line 1, in <lambda> attributeerror: 'nonetype' object has no attribute 'extend' 

flat_list = [item sublist in l item in sublist] 

which means:

for sublist in l:     item in sublist:         flat_list.append(item) 

is faster shortcuts posted far. (l list flatten.)

here corresponding function:

flatten = lambda l: [item sublist in l item in sublist] 

for evidence, always, can use timeit module in standard library:

$ python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' '[item sublist in l item in sublist]' 10000 loops, best of 3: 143 usec per loop $ python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'sum(l, [])' 1000 loops, best of 3: 969 usec per loop $ python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'reduce(lambda x,y: x+y,l)' 1000 loops, best of 3: 1.1 msec per loop 

explanation: shortcuts based on + (including implied use in sum) are, of necessity, o(l**2) when there l sublists -- intermediate result list keeps getting longer, @ each step new intermediate result list object gets allocated, , items in previous intermediate result must copied on (as few new ones added @ end). (for simplicity , without actual loss of generality) have l sublists of items each: first items copied , forth l-1 times, second items l-2 times, , on; total number of copies times sum of x x 1 l excluded, i.e., i * (l**2)/2.

the list comprehension generates 1 list, once, , copies each item on (from original place of residence result list) once.