I have a list of lists like
[
[1, 2, 3],
[4, 5, 6],
[7],
[8, 9]
]
How can I flatten it to get [1, 2, 3, 4, 5, 6, 7, 8, 9]?
If your list of lists comes from a nested list comprehension, the problem can be solved more simply/directly by fixing the comprehension; please see How can I get a flat result from a list comprehension instead of a nested list?.
The most popular solutions here generally only flatten one "level" of the nested list. See Flatten an irregular (arbitrarily nested) list of lists for solutions that completely flatten a deeply nested structure (recursively, in general).
A list of lists named xss can be flattened using a nested list comprehension:
flat_list = [
x
for xs in xss
for x in xs
]
The above is equivalent to:
flat_list = []
for xs in xss:
for x in xs:
flat_list.append(x)
Here is the corresponding function:
def flatten(xss):
return [x for xs in xss for x in xs]
This is the fastest method.
As evidence, using the timeit module in the standard library, we see:
$ python -mtimeit -s'xss=[[1,2,3],[4,5,6],[7],[8,9]]*99' '[x for xs in xss for x in xs]'
10000 loops, best of 3: 143 usec per loop
$ python -mtimeit -s'xss=[[1,2,3],[4,5,6],[7],[8,9]]*99' 'sum(xss, [])'
1000 loops, best of 3: 969 usec per loop
$ python -mtimeit -s'xss=[[1,2,3],[4,5,6],[7],[8,9]]*99' 'reduce(lambda xs, ys: xs + ys, xss)'
1000 loops, best of 3: 1.1 msec per loop
Explanation: the methods 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 M items each: the first M items are copied back and forth L-1 times, the second M items L-2 times, and so on; total number of copies is M times the sum of x for x from 1 to L excluded, i.e., M * (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.