combine lists in list/dict comprehension way

Question

is it possible to apply list/dictionary comprehension to the following code to have ["abc", "ab", "cd"]

tk = {}
tk[1] = ["abc"]
tk[2] = ["ab", "cd"]
tk[3] = ["ef", "gh"]
t = (1, 2)
combined = []
combined.append(tk[i]) for i in t #does not work. note, not all tk values are used, this is based on t.

I could think of ll = [tk[i] for i in t], then this turns to be flatten list out of lists. so

ll = [tk[i] for i in t]
[item for sublist in ll for item in sublist]

but this is not one-liner. I wonder if there is better way.

Why do you need a list comprehension when you can just do: `tk[1] + tk[2]`? — Moinuddin Quadri, Apr 04 '17 at 23:01
You should use `extend` instead of `append`, and you may benefit from `itertools.chain`. Also pay attention at braces — Andrea Corbellini, Apr 04 '17 at 23:03
this is only an illustrative example. I have a long sequence to iterate. — pepero, Apr 04 '17 at 23:04
This is typically referred to as list flattening. http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python — Peter Gibson, Apr 04 '17 at 23:04
@Meccano do *NOT* use list-comprehensions with side-effects. You can simply do `[s for v in tk.values() for s in v]`, anyway (order not guaranteed). — juanpa.arrivillaga, Apr 04 '17 at 23:08

Moinuddin Quadri · Answer 1 · 2017-04-04T23:10:43.067

2

If the order of values in the desired list matters, the generalized way to achieve this would be to sort the items in the dict based on the key, and merge the list of values. For example:

>>> from operator import itemgetter
>>> from itertools import chain

>>> list(chain.from_iterable(i for _, i in sorted(tk.items(), key=itemgetter(0))))
['abc', 'ab', 'cd']

edited Apr 04 '17 at 23:10

answered Apr 04 '17 at 23:09

Moinuddin Quadri

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One should prefer `chain.from_iterable` over splatting: http://stackoverflow.com/questions/39533052/difference-between-chainiter-vs-chain-from-iterableiter – juanpa.arrivillaga Apr 04 '17 at 23:10
@juanpa.arrivillaga Thanks for pointing that out. I should have used `chain.from_iterable` at the first place – Moinuddin Quadri Apr 04 '17 at 23:12
1

Yep. I used to prefer the *look* of the splatting version, until I thought about it and realized that it would have to consume the iterable into a tuple and process it from there! – juanpa.arrivillaga Apr 04 '17 at 23:13

Matheus Portela · Answer 2 · 2017-04-05T02:41:43.977

Simply iterate through the values of your dictionary. Something like this:

>>> tk = {}
>>> tk[1] = ['abc']
>>> tk[2] = ['ab', 'cd']
>>> combined = [x for t in tk.values() for x in t]
>>> print combined
['abc', 'ab', 'cd']

You can even use an OrderedDict if you need to maintain the order of your lists, since a regular dict does not guarantee the order of its keys:

>>> import collections
>>> tk = collections.OrderedDict()
>>> tk[1] = ['abc']
>>> tk[2] = ['ab', 'cd']
>>> combined = [x for t in tk.values() for x in t]
>>> print combined
['abc', 'ab', 'cd']

Note that [tk[i] for i in (1, 2)], as you proposed, won't have the desired results. You still need to iterate through the values inside each list.

>>> [tk[i] for i in (1, 2)]
[['abc'], ['ab', 'cd']]

Also note that [tk[i] for i in tk], as you proposed later, is exactly the same as tk.values(). So you could the proposed solution [x for t in tk.values() for x in t] is equivalent to what you achieved but in one line.

score 1 · Answer 3 · answered Apr 05 '17 at 10:23

1

Given your constraint of choosing a sequence of keys manually:

>>> tk = {}
>>> tk[1] = ["abc"]
>>> tk[2] = ["ab", "cd"]
>>> tk[3] = ["ef", "gh"]

You want:

>>> [vals for i in (1,2) for vals in tk[i]]
['abc', 'ab', 'cd']

answered Apr 05 '17 at 10:23

juanpa.arrivillaga

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combine lists in list/dict comprehension way

3 Answers3