How to convert a nested list into a one-dimensional list in Python?

If you're using python < 3 then you can do the following:

from compiler.ast import flatten
list = [1,[2,2,2],4]
print flatten(list)

The manual equivalent in python 3.0 would be (taken from this answer):

def flatten(x):
    result = []
    for el in x:
        if hasattr(el, "__iter__") and not isinstance(el, str):
            result.extend(flatten(el))
        else:
            result.append(el)
    return result

 print(flatten(["junk",["nested stuff"],[],[[]]]))  

You could even do the same in a list comprehension:

list = [1,[2,2,2],4]
l = [item for sublist in list for item in sublist]

Which is the equivalent of:

l = [[1], [2], [3], [4], [5]]
result = []
for sublist in l:
    for item in sublist:
        result.append(item)

print(result)

from nltk import flatten

example_list = [1, [2, 3], 3]
flattened_list = flatten(example_list)
print(flattened_list)

Output: [1, 2, 3, 3]

To Make A Single list from a Nested List in python we can simply do this :

from functools import reduce

some_list = [[14], [215, 383, 87], [298], [374], [2,3,4,5,6,7]]
single_list = reduce(lambda x,y: x+y, some_list)
print(single_list)

Output: [14, 215, 383, 87, 298, 374, 2, 3, 4, 5, 6, 7]

You need to recursively loop over the list and check if an item is iterable(strings are iterable too, but skip them) or not.

itertools.chain will not work for [1,[2,2,2],4] because it requires all of it's items to be iterable, but 1 and 4 (integers) are not iterable. That's why it worked for the second one because it's a list of lists.

>>> from collections import Iterable
def flatten(lis):
     for item in lis:
         if isinstance(item, Iterable) and not isinstance(item, str):
             for x in flatten(item):
                 yield x
         else:        
             yield item

>>> lis = [1,[2,2,2],4]
>>> list(flatten(lis))
[1, 2, 2, 2, 4]
>>> list(flatten([[1, 2, 3], [4, 5, 6], [7, 8, 9]]))
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Works for any level of nesting:

>>> a = [1,[2,2,[2]],4]
>>> list(flatten(a))
[1, 2, 2, 2, 4]

Unlike other solutions, this will work for strings as well:

>>> lis = [1,[2,2,2],"456"]
>>> list(flatten(lis))
[1, 2, 2, 2, '456']