How to completely flatten a list in Python?

Question

Flattening lists of lists is a well-known problem but how does one flatten a list of lists of lists in the most pythonic way? My ultimate goal is to create a numpy array as follows:

[In]: mylist = [[[1,2,3], [4]], [[5,6], [7,8]]]
[Out]: array([[1, 2, 3, 4],
              [5, 6, 7, 8]])

but even just completely flattening into a long list will do (can be easily converted into the right-shaped array anyway). A typical output array will have dimensions (10000, 10) so looping through the rows-to-be is very inefficient. All ideas will be appreciated!

(edit) Best solution I can think of:

import itertools

for i in range(len(mylist)):
    mylist[i] = list(itertools.chain.from_iterable(mylist[i]))
np.array(mylist)

Just to understand the problem how could you have a looping solution to solve your sample case? — Divakar, Aug 25 '18 at 12:57
In fact many solutions which look tempting at first return `array([list([1, 2, 3]), list([4]), list([5, 6]), list([7, 8])], dtype=object)` — coaxialquantum, Aug 25 '18 at 13:07
This is also one of my favourites: https://stackoverflow.com/a/10632307/7919597 — Joe, Aug 25 '18 at 13:20
`list(map(list, map(itertools.chain.from_iterable, mylist)))`? — Mark Dickinson, Aug 25 '18 at 13:32

Dani Mesejo · Answer 1 · 2018-08-25T13:59:46.997

0

A possibility is to use numpy.append:

import numpy as np

rows = [[[1, 2, 3], [4]], [[5, 6], [7, 8]]]
result = np.array([np.append(value[0], value[1:]) for value in rows])

print(result)

Output

[[1 2 3 4]
 [5 6 7 8]]

The function numpy.append flattens value[1:]. One alternative is to use concatenate and ravel:

rows = [[[1], [2], [3]], [[4], [5], [6]]]
result = np.array([np.concatenate(value).ravel() for value in rows])
print(result)

Output

[[1 2 3]
 [4 5 6]]

The concatenate function join a sequence of arrays and np.ravel returns a contiguous flattened array. You could change np.ravel by np.flatten.

edited Aug 25 '18 at 13:59

answered Aug 25 '18 at 13:17

Dani Mesejo

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Nice idea but this fails when the rows have more than two lists in them... `rows = [[[1], [2], [3]], [[4], [5], [6]]]` throws a TypeError. – coaxialquantum Aug 25 '18 at 13:30
append does a lot of relocation. It is not amortized at all, unlike list append and extended. Still a good thought. – Mad Physicist Aug 25 '18 at 13:33
Updated my answer – Dani Mesejo Aug 25 '18 at 14:00

How to completely flatten a list in Python?

1 Answers1