One-liner to merge lists?

[MRAB]

On 2022-02-27 08:51, Barry Scott wrote:
> 
> 
>> On 22 Feb 2022, at 09:30, Chris Angelico <rosuav at gmail.com> wrote:
>> 
>> On Tue, 22 Feb 2022 at 20:24, Frank Millman <frank at chagford.com <mailto: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.
> 
> 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?