Python – Flatten List to individual elements

Sometimes, while working with a Python list, we can have a problem in which we need to perform flatten of the list, i.e convert a mixed list to a flattened one. This can have applications in domains that use 1D lists as input. Let’s discuss certain ways in which this task can be performed.

Flatten List using list comprehension

Here, we are using list comprehension to flatten the list from 2D to 1D.

Python3

res = [i for row in [[1,3,"geeks"], [4,5],
                     [6,"best"]] for i in row]
print(res)

Output:

[1, 3, 'geeks', 4, 5, 6, 'best']

Flatten List using sum()

Here, we are using sum() function in which we passed test_list as iterable object as first parameter, and second parameter as empty list in which it stores the element.

Python3

test_list = [[1,3,"gfg"], [4,5], [6,"best"]]

test_list = sum(test_list, [])

print(test_list)

Output:

[1, 3, 'gfg', 4, 5, 6, 'best']

Flatten List using loop

The combination of the above functionalities can be used to perform this task. In this, we check for an instance of the list and flatten it and the rest of the elements we add to the list brutely. 

Python3

def flatten(test_list):
    if isinstance(test_list, list):
        temp = []
        for ele in test_list:
            temp.extend(flatten(ele))
        return temp
    else:
        return [test_list]

# Initializing list
test_list = ['gfg', 1, [5, 6, 'geeks'], 67.4, [5], 'best']

# Flatten List to individual elements
# using loop + isinstance()
res = flatten(test_list)

# printing result
print ("The List after flattening : " + str(res))

Output:

The List after flattening : [‘gfg’, 1, 5, 6, ‘geeks’, 67.4, 5, ‘best’]

Flatten List using flatten() method

Pandas flatten return a copy of the array collapsed into one dimension.

Python3

from pandas.core.common import flatten

l = [[1,3,"gfg"], [4,5], [6,"best"]]
print(list(flatten(l)))

Output:

[1, 3, 'gfg', 4, 5, 6, 'best']

Flatten List using chain() + isinstance()

This is yet another way in which this task can be performed. In this we perform the task of iteration using chain() and checking for list instance is done using isinstance(). 

Python3

from itertools import chain

# Initializing list
test_list = ['gfg', 1, [5, 6, 'geeks'], 67.4, [5], 'best']

# Flatten List to individual elements
# using chain() + isinstance()
res = list(chain(*[ele if isinstance(ele, list)
                   else [ele] for ele in test_list]))

# printing result
print ("The List after flattening : " + str(res))

Output :

The List after flattening : [‘gfg’, 1, 5, 6, ‘geeks’, 67.4, 5, ‘best’]

Flatten List using reduce() function
The reduce() function is defined in the “functools” module. It applies a function of two arguments continuously on the given sequence and returns a single value.

Python3

from functools import reduce

# Initializing list
test_list = [[1,3,"gfg"], [4,5], [6,"best"]]

# Flatten List to individual elements
# using reduce()
res = reduce(lambda x,y: x+y, test_list)

# printing result
print ("The List after flattening : " + str(res))
#This code is contributed by Edula Vinay Kumar Reddy

Output

The List after flattening : [1, 3, 'gfg', 4, 5, 6, 'best']

Time complexity: O(n)

Auxiliary Space: O(n)