A list of string into a nested list of string

Question

so I need to turn a list of str into a list of list of str.

EX:

thing = [' a b c', 'e f g']

into:

[['a', 'b', 'c'], ['e', 'f', 'g']]

My code keeps returning a mistake:

coolthing = []   
abc = []
for line in thing:
    abc = coolthing.append(line.split())
return abc

user2555451 · Accepted Answer · 2015-04-03 19:38:01Z

3

list.append operates in-place and always returns None. So, abc will be None when you return it.

To do what you want, you can use a list comprehension:

return [x.split() for x in thing]

Demo:

>>> thing = [' a b c', 'e f g']
>>> [x.split() for x in thing]
[['a', 'b', 'c'], ['e', 'f', 'g']]

answered Apr 3, 2015 at 19:38

user2555451

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5 Comments

Padraic Cunningham Over a year ago

return map(str.split,thing) for a functional apporach

user2555451 Over a year ago

@PadraicCunningham - +1. I actually thought about doing that initially, but then you'd have to do return list(map(str.split,thing)) if the OP was using Python 3.x. Nice thing about the list comp. is it works the same in both versions.

Padraic Cunningham Over a year ago

Your way is probably better for the OP, just thought I would throw it in there.

pzp Over a year ago

Based on my rudimentary testing with timeit, the functional approach is nominally faster.

user2555451 Over a year ago

@pzp1997 - True. In Python 2.x, the map approach will be faster because most of the work (i.e. the looping) is being done in C and not Python. Note however that in Python 3.x, the list comp. outperforms list(map(str.split,thing)). Mainly though, I used the list comp. for portability. It also seems slightly more readable to me.

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