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).
Best Answer
A list of lists named
xss
can be flattened using a nested list comprehension:The above is equivalent to:
Here is the corresponding function:
This is the fastest method. As evidence, using the
timeit
module in the standard library, we see:Explanation: the methods based on
+
(including the implied use insum
) 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 forthL-1
times, the second M itemsL-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.