Python | Ways to flatten a 2D list

Given a 2D list, Write a Python program to convert the given list into a flattened list. 

Method #1: Using chain.iterable()

Python3

# Python code to demonstrate
# converting 2d list into 1d list
# using chain.from_iterables

# import chain
from itertools import chain

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

# printing initial list
print ("initial list ", str(ini_list))

# converting 2d list into 1d
# using chain.from_iterables
flatten_list = list(chain.from_iterable(ini_list))

# printing flatten_list
print ("final_result", str(flatten_list))

Output:

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

Method #2: Using list comprehension

Python3

# Python code to demonstrate
# converting 2d list into 1d list
# using list comprehension

# import chain
from itertools import chain

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

# printing initial list
print ("initial list ", str(ini_list))

# converting 2d list into 1d
# using list comprehension
flatten_list = [j for sub in ini_list for j in sub]

# printing flatten_list
print ("final_result", str(flatten_list))

Output:

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

Method #3: Using functools.reduce

Python3

# Python code to demonstrate
# converting 2d list into 1d list
# using functools.reduce

# import functools
from functools import reduce

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

# printing initial list
print ("initial list ", str(ini_list))

# converting 2d list into 1d
# using functools.reduce
flatten_list = reduce(lambda z, y :z + y, ini_list)

# printing flatten_list
print ("final_result", str(flatten_list))

Output:

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

Method #4: Using sum
sum has an optional argument: sum(iterable [, start])

Python3

# Python code to demonstrate
# converting 2d list into 1d list
# using sum

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

# printing initial list
print ("initial list ", str(ini_list))

# converting 2d list into 1d
flatten_list = sum(ini_list, [])

# printing flatten_list
print ("final_result", str(flatten_list))

# This code is contributed by
# Mayank Chaudhary - chaudhary_19

Output:

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

Method #5: Using lambda

Python3

#Python 3 code to flatten nested list
#contributed by S Lakshman Rao - kaapalx
ini_list=[[1, 2, 3],
          [3, 6, 7],
          [7, 5, 4]]

#Using lambda

flatten_list = lambda y:[x for a in y for x in flatten_list(a)] if type(y) is list else [y]

print("Initial list ",ini_list) #printing initial list

print("Flattened List ",flatten_list(ini_list)) # printing flattened list

Output:

Initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened List  [1, 2, 3, 3, 6, 7, 7, 5, 4]

Method #6: Using numpy

Python3

#Python 3 code to flatten nested list
#Contributed by S Lakshman Rao - kaapalx
import numpy

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

print("Initial list ",ini_list) #Printing Initial list

print("Flattened List ",list(numpy.concatenate(ini_list).flat))
#Using numpy to flatten list and printing the result

Output:

Initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened List  [1, 2, 3, 3, 6, 7, 7, 5, 4]

Method #7: Using extend() method

Python3

#Python 3 code to flatten nested list

ini_list=[[1, 2, 3],[3, 6, 7],[7 ,5, 4]]
print("Initial list ",ini_list) #Printing Initial list
res=[]
for i in ini_list:
    res.extend(i)
print("Flattened List ",res)

Output

Initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened List  [1, 2, 3, 3, 6, 7, 7, 5, 4]