[Fixed] How do I make a flat list out of a list of lists in python?

To flatten a list of lists xss:

flat_list = [x for xs in xss for x in xs]

This is equivalent to:

flat_list = []
for xs in xss:
    for x in xs:
        flat_list.append(x)

Or as a function:

def flatten(xss):
    return [x for xs in xss for x in xs]

Performance analysis:

To measure performance, we use the timeit module from the standard library:

$ python -mtimeit -s't=[[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't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'sum(t, [])'
1000 loops, best of 3: 969 usec per loop
$ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'reduce(lambda x,y: x+y,t)'
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(T**2) when there are T 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 T sublists of k items each: the first k items are copied back and forth T-1 times, the second k items T-2 times, and so on; total number of copies is k times the sum of x for x from 1 to T excluded, i.e., k * (T**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.