One-liner to merge lists?

[Avi Gross]

Frank,

Given this kind of data: d = {1: ['aaa', 'bbb', 'ccc'], 2: ['fff', 'ggg']}

You seem focused on a one-liner, however ridiculous or inefficient. Does this count as a somewhat weird one liner?

comb = [];   _ = [comb.append(v) for v in d.values()]

If you run the code and ignore the returned result, the value in comb is:

print(comb)

[['aaa', 'bbb', 'ccc'], ['fff', 'ggg']]

As discussed the other day here, this is an example of a variant on the anonymous function discussion as what is wanted is a side effect. I could omit assigning the result to anything and run this code after the previous like this:

[comb.append(v) for v in d.values()]
[None, None]
comb
[['aaa', 'bbb', 'ccc'], ['fff', 'ggg'], ['aaa', 'bbb', 'ccc'], ['fff', 'ggg']]

As shown the list comprehension itself is not returning anything of value and need not be assigned to anything. But it then generally is set to autoprint and that can be supressed by assigning the value to _ or anything you can ignore.

I am NOT saying this is a good way but that it is sort of a one-liner. There are good reasons I do not see people doing things this way!

-----Original Message-----
From: Frank Millman <frank at chagford.com>
To: python-list at python.org
Sent: Tue, Feb 22, 2022 4:19 am
Subject: One-liner to merge lists?


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?



Thanks



Frank Millman





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