One-liner to merge lists?

[Chris Angelico]

On Tue, 22 Feb 2022 at 20:46, Frank Millman <frank at chagford.com> wrote:
>
> On 2022-02-22 11:30 AM, Chris Angelico wrote:
> > On Tue, 22 Feb 2022 at 20:24, Frank Millman <frank at 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.
> >
>
> Thanks, that is neat.
>
> However, I did see this -
>
>  >>> help(sum)
> Help on built-in function sum in module builtins:
>
> sum(iterable, /, start=0)
>      Return the sum of a 'start' value (default: 0) plus an iterable of
> numbers
>
>      When the iterable is empty, return the start value.
>      This function is intended specifically for use with numeric values
> and may reject non-numeric types.
>  >>>
>
> So it seems that it is not recommended.
>
> I think I will stick with itertools.chain.
>

Yup, itertools.chain is definitely the recommended way to do things.
It's short, efficient, and only slightly unclear. If the clarity is a
problem, you can always wrap it into a function:

def sum_lists(iterable):
    return list(chain.from_iterable(iterable))


ChrisA