Ronald Oussoren added the comment: Appending a sequence of lists with sum is inefficient because it (currently) does a lot of copying, and that gets noticable when you sum a larger number of lists
Note how using sum for add 200 lists is more than twice as long as adding 100 lists: ronald@gondolin[0]$ python -m timeit -s "lists=[['a']*100 for i in range(100)]" "sum(lists, [])" 100 loops, best of 3: 2.04 msec per loop ronald@gondolin[0]$ python -m timeit -s "lists=[['a']*100 for i in range(200)]" "sum(lists, [])" 100 loops, best of 3: 9.2 msec per loop Also note how using itertools.chain is both a lot faster and behaves better: ronald@gondolin[0]$ python -m timeit -s "import itertools; lists=[['a']*100 for i in range(100)]" "list(itertools.chain.from_iterable(lists))" 10000 loops, best of 3: 165 usec per loop ronald@gondolin[0]$ python -m timeit -s "import itertools; lists=[['a']*100 for i in range(100)]" "list(itertools.chain.from_iterable(lists))" 10000 loops, best of 3: 155 usec per loop (I used python2.7 for this, the same behavior can be seem with python 3). See also #18305, which proposed a small change to how sum works which would fix the performance problems for summing a sequence of lists (before going too far and proposing to add special-case tuples and string) ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue18424> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
