On 22/02/2022 10:44, Frank Millman wrote:
On 2022-02-22 11:30 AM, Chris Angelico wrote:
On Tue, 22 Feb 2022 at 20:24, Frank Millman <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.
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.
If you don't like the idea of 'adding' strings you can 'concat'enate:
>>> items = [[1,2,3], [4,5], [6]]
>>> functools.reduce(operator.concat, items)
[1, 2, 3, 4, 5, 6]
>>> functools.reduce(operator.iconcat, items, [])
[1, 2, 3, 4, 5, 6]
The latter is the functional way to spell your for... extend() loop.
Don't forget to provide the initial value in that case lest you modify
the input:
>> functools.reduce(operator.iconcat, items) # wrong
[1, 2, 3, 4, 5, 6]
>>> items
[[1, 2, 3, 4, 5, 6], [4, 5], [6]] # oops
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