On Mon, 28 Feb 2022 at 03:24, MRAB <pyt...@mrabarnett.plus.com> wrote: > > On 2022-02-27 08:51, Barry Scott wrote: > > > > > >> On 22 Feb 2022, at 09:30, Chris Angelico <ros...@gmail.com> wrote: > >> > >> On Tue, 22 Feb 2022 at 20:24, Frank Millman <fr...@chagford.com > >> <mailto:fr...@chagford.com>> wrote: > >>> > >>> Hi all > >>> > >>> I think this should be a simple one-liner, but I cannot figure it out. > >>> > >>> I have a dictionary with a number of keys, where each value is a single > >>> list - > >>> > >>>>>> d = {1: ['aaa', 'bbb', 'ccc'], 2: ['fff', 'ggg']} > >>> > >>> I want to combine all values into a single list - > >>> > >>>>>> ans = ['aaa', 'bbb', 'ccc', 'fff', 'ggg'] > >>> > >>> I can do this - > >>> > >>>>>> a = [] > >>>>>> for v in d.values(): > >>> ... a.extend(v) > >>> ... > >>>>>> a > >>> ['aaa', 'bbb', 'ccc', 'fff', 'ggg'] > >>> > >>> I can also do this - > >>> > >>>>>> from itertools import chain > >>>>>> a = list(chain(*d.values())) > >>>>>> a > >>> ['aaa', 'bbb', 'ccc', 'fff', 'ggg'] > >>>>>> > >>> > >>> Is there a simpler way? > >>> > >> > >> itertools.chain is a good option, as it scales well to arbitrary > >> numbers of lists (and you're guaranteed to iterate over them all just > >> once as you construct the list). But if you know that the lists aren't > >> too large or too numerous, here's another method that works: > >> > >>>>> sum(d.values(), []) > >> ['aaa', 'bbb', 'ccc', 'fff', 'ggg'] > >> > >> It's simply adding all the lists together, though you have to tell it > >> that you don't want a numeric summation. > > > > If you code is performance sensitive do not use sum() as it creates lots of > > tmp list that are deleted. > > > > I have an outstanding ticket at work to replace all use of sum() on lists > > as when we profiled it > > stands out as a slow operation. We have a lots of list of list that we need > > to flatten. > > > I think that 'sum' uses '__add__' but not '__iadd__'. > > If it copied the given value, if present, and then used '__iadd__', if > present, wouldn't that speed it up?
It's hardly relevant. If you care about speed, use chain(), like everyone's been saying all along :) ChrisA -- https://mail.python.org/mailman/listinfo/python-list
