Given a list of lists
flat_list = [item for sublist in l for item in sublist]
flat_list = 
for sublist in l:
for item in sublist:
is faster than the shortcuts posted so far. (
l is the list to flatten.)
Here is the corresponding function:
flatten = lambda l: [item for sublist in l for item in sublist]
As evidence, you can use the
timeit module in the standard library:
$ python -mtimeit -s'l=[[1,2,3],[4,5,6], , [8,9]]*99' '[item for sublist in l for item in sublist]'
10000 loops, best of 3: 143 usec per loop
$ python -mtimeit -s'l=[[1,2,3],[4,5,6], , [8,9]]*99' 'sum(l, )'
1000 loops, best of 3: 969 usec per loop
$ python -mtimeit -s'l=[[1,2,3],[4,5,6], , [8,9]]*99' 'reduce(lambda x,y: x+y,l)'
1000 loops, best of 3: 1.1 msec per loop
Explanation: the shortcuts based on
+ (including the implied use in
sum ) are, of necessity,
O(L**2) when there are L sublists – as the intermediate result list keeps getting longer, at each step a new intermediate result list object gets allocated, and all the items in the previous intermediate result must be copied over (as well as a few new ones added at the end). So, for simplicity and without actual loss of generality, say you have L sublists of I items each: the first I items are copied back and forth L-1 times, the second I items L-2 times, and so on; total number of copies is I times the sum of x for x from 1 to L excluded, i.e.,
I * (L**2)/2 .
The list comprehension just generates one list, once, and copies each item over (from its original place of residence to the result list) also exactly once.