Python Program to Flatten a Nested List

Example 1: Using List Comprehension

my_list = [[1], [2, 3], [4, 5, 6, 7]]

flat_list = [num for sublist in my_list for num in sublist]
print(flat_list)

Output

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

This is one of the simplest pythonic ways of flattening a list.

• Using list comprehension access the sublist from my_list, then access each element of the sublist.
• Each element num is stored in flat_list.

Learn more about list comprehension at Python List Comprehension.

Example 2: Using Nested for Loops (non pythonic way)

my_list = [[1], [2, 3], [4, 5, 6, 7]]

flat_list = []
for sublist in my_list:
    for num in sublist:
        flat_list.append(num)

print(flat_list)

Output

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

• Create an empty list flat_list.
• Access each element of the sublist using a nested loop and append that element to flat_list.

Example 3: Using itertools package

import itertools

my_list = [[1], [2, 3], [4, 5, 6, 7]]

flat_list = list(itertools.chain(*my_list))
print(flat_list)

Output

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

Using itertools module, we can create a flattened list.

• chain() method from itertools module returns each element of each iterable (i.e. sub lists ).
• list() converts those returned values into a list.

Example 4: Using sum()

my_list = [[1], [2, 3], [4, 5, 6, 7]]

flat_list = sum(my_list, [])
print(flat_list)

Output

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

• Provide two arguments to the sum() method: my_list and an empty list (i.e. [ ]).
• sum() combines my_list and [ ] to produce a flattened list.

Example 5: Using lambda and reduce()

from functools import reduce

my_list = [[1], [2, 3], [4, 5, 6, 7]]
print(reduce(lambda x, y: x+y, my_list))

Output

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

In the above example, reduce() applies the lambda function to all the elements of my_list.

Learn more about lambda expressions at Python Anonymous/Lambda Function.