How to flatten a hetrogenous list of list into a single list in python?

Question

I have a list of objects where objects can be lists or scalars. I want an flattened list with only scalars. Eg:

L = [35,53,[525,6743],64,63,[743,754,757]]
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Answer 1

Here is a relatively simple recursive version which will flatten any depth of list

l = [35,53,[525,6743],64,63,[743,754,757]]

def flatten(xs):
    result = []
    if isinstance(xs, (list, tuple)):
        for x in xs:
            result.extend(flatten(x))
    else:
        result.append(xs)
    return result

print flatten(l)

Answer 2

l = [35,53,[525,6743],64,63,[743,754,757]]
outputList = []

for i in l:
    if isinstance(i, list):
        outputList.extend(i)
    else:
        outputList.append(i)

Answer 3

The answer is quite simple. Take advantage of recursion.

def flatten(nst_lst, final_list):

    for val in nst_lst:
        if isinstance(val, list):
            flatten(val, final_list)
        else:
            final_list.append(val)
    return final_list

#Sample usage
fl_list = []
lst_to_flatten = [["this",["a",["thing"],"a"],"is"],["a","easy"]]

print(flatten(lst_to_flatten, fl_list))

Answer 4

This solution is only for your specific situation (scalars within lists) and assumes the scalars are integer. It is a terrible solution but it is incredibly short.

outputlist = map(int,",".split(str(L).replace("[","").replace("]","")))

Answer 5

it could be done neatly in one line using numpy

import numpy as np
np.hstack(l)

you end up with an ndarray

array([  35,   53,  525, 6743,   64,   63,  743,  754,  757])

Answer 6

Here's a oneliner, based on the question you've mentioned:

list(itertools.chain(*((sl if isinstance(sl, list) else [sl]) for sl in l)))

UPDATE: And a fully iterator-based version:

from itertools import imap, chain
list(chain.from_iterable(imap(lambda x: x if isinstance(x, list) else [x], l)))