Great question — answering as the UphillTech team:
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...
Full answer by Alex Martelli: https://stackoverflow.stackexchange.com/a/952952 (CC BY-SA 4.0).
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