How to unnest a nested list

Question

Given a multidimensional (nested) Python list, how can I easily get a single list that contains all the elements of its sub-lists?

For example, given [[1,2,3,4,5]] I want to get [1,2,3,4,5]; similarly [[1,2], [3,4]] should become [1,2,3,4].

`[item for sublist in l for item in sublist]`. This is a duplicate of a number of other questions. See, for example, http://stackoverflow.com/q/952914/623518, http://stackoverflow.com/q/406121/623518 and http://stackoverflow.com/q/457215/623518. — Chris, Aug 08 '12 at 08:25
@zrxq It isn't that question but it is a duplicate of the ones mentioned by Chris — jamylak, Aug 08 '12 at 08:27

score 85 · Answer 1 · edited Jun 20 '20 at 09:12

85

Use itertools.chain:

itertools.chain(*iterables):

Make an iterator that returns elements from the first iterable until it is exhausted, then proceeds to the next iterable, until all of the iterables are exhausted. Used for treating consecutive sequences as a single sequence.

Example:

from itertools import chain

A = [[1,2], [3,4]]

print list(chain(*A))
# or better: (available since Python 2.6)
print list(chain.from_iterable(A))

The output is:

[1, 2, 3, 4]
[1, 2, 3, 4]

edited Jun 20 '20 at 09:12

Community

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answered Aug 08 '12 at 08:30

moooeeeep

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It useful to clarify that all objects within the list must be lists themselves. For example, if 'A= ['year', [1] ]' then 'chain' will not produce the expected results. It requires 'A = [['year'], [1]]'. – msh855 Mar 20 '19 at 12:30
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@msh855 A string is an iterable too. So, in this case, it seems to be more an issue with the expectation, rather than with the result. – moooeeeep Mar 20 '19 at 14:38
Have you tried it, or is indeed a matter of expectation?. I tried this: A= ['year', [1,2,'bad']] list(chain(*A)) list(chain.from_iterable(A)) and produced: Out[153]: ['y', 'e', 'a', 'r', 1, 2, 'bad'] – msh855 Mar 20 '19 at 14:58
You pass two iterables, both of which are iterated. The first produces the characters in the string, the second produces the list elements. If you need a different behavior, you might need to write specific code. – moooeeeep Mar 20 '19 at 17:09

lollo · Answer 2 · 2014-01-10T09:57:53.320

42

Use reduce function

reduce(lambda x, y: x + y, A, [])

Or sum

sum(A, [])

edited Jan 10 '14 at 09:57

answered Aug 08 '12 at 08:25

lollo

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`reduce` was removed in Python 3. The `sum` version works but the `itertools.chain` method is much faster. – Boris Verkhovskiy Nov 24 '19 at 22:41

score 7 · Answer 3 · answered Aug 08 '12 at 08:25

7

the first case can also be easily done as:

A=A[0]

answered Aug 08 '12 at 08:25

codious

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the OP wrote that `[[1,2], [3,4]]` should turn into `[1,2,3,4]`. Your solution returns `[1,2]`! – Qaswed Mar 06 '20 at 13:03

score 6 · Answer 4 · answered Aug 08 '12 at 08:29

itertools provides the chain function for that:

From http://docs.python.org/library/itertools.html#recipes:

def flatten(listOfLists):
    "Flatten one level of nesting"
    return chain.from_iterable(listOfLists)

Note that the result is an iterable, so you may need list(flatten(...)).

How to unnest a nested list

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