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Python - Flatten a Nested List

Last Updated : May 01, 2025

In Python, flattening a nested list means converting a list of lists into a single list with all the elements. In this chapter, we will learn different ways to flatten nested lists using loops, list comprehensions, and built-in functions with the help of examples.

Flatten a Nested List Using a For Loop

You can flatten a nested list by iterating over each sublist and then over each item in the sublist.

Note: This method works for lists with one level of nesting.

Example

In this example, we are flattening a nested list of city names using a simple for loop.

# Creating a nested list of cities
nested_cities = [
    ["New York", "London"],
    ["Paris", "Tokyo"],
    ["Sydney", "Dubai"]]

# Flattening the list using a for loop
flat_list = []
for sublist in nested_cities:
    for city in sublist:
        flat_list.append(city)

# Displaying the flattened list
print(flat_list)

The output of the above code would be:

['New York', 'London', 'Paris', 'Tokyo', 'Sydney', 'Dubai']

Flatten a Nested List Using List Comprehension

List comprehension provides a concise way to flatten a nested list in a single line of code. This is suitable for lists with a single level of nesting.

Example

In this example, we are flattening the nested list of cities using list comprehension.

# Creating a nested list of cities
nested_cities = [
    ["New York", "London"],
    ["Paris", "Tokyo"],
    ["Sydney", "Dubai"]]

# Flattening the list using list comprehension
flat_list = [city for sublist in nested_cities for city in sublist]

# Displaying the flattened list
print(flat_list)

The output of the above code would be:

['New York', 'London', 'Paris', 'Tokyo', 'Sydney', 'Dubai']

Flatten a Nested List Using itertools.chain()

The itertools.chain() function from the Python standard library can also be used to flatten a nested list.

Example

In this example, we are using itertools.chain() to flatten the nested list of cities.

# Importing chain from itertools
from itertools import chain

# Creating a nested list of cities
nested_cities = [
    ["New York", "London"],
    ["Paris", "Tokyo"],
    ["Sydney", "Dubai"]]

# Flattening the list using itertools.chain
flat_list = list(chain.from_iterable(nested_cities))

# Displaying the flattened list
print(flat_list)

The output of the above code would be:

['New York', 'London', 'Paris', 'Tokyo', 'Sydney', 'Dubai']

Flatten a Nested List Using Recursion

For lists with multiple levels of nesting, you can use recursion to flatten them completely.

Example

In this example, we are using a recursive function to flatten a list that contains deeply nested city names.

# Defining a recursive function to 
# flatten any level of nested list
def flatten_list(nested_list):
    flat_list = []
    for item in nested_list:
        if isinstance(item, list):
            flat_list.extend(flatten_list(item))
        else:
            flat_list.append(item)
    return flat_list

# Creating a deeply nested list of cities
nested_cities = [["New York", ["London", ["Paris"]]], ["Tokyo", "Sydney"], "Dubai"]

# Flattening the list using recursion
flat_list = flatten_list(nested_cities)

# Displaying the flattened list
print(flat_list)

The output of the above code would be:

['New York', 'London', 'Paris', 'Tokyo', 'Sydney', 'Dubai']

Flatten a Nested List Using numpy

If your nested list contains numeric values, you can use the numpy library to flatten it easily using numpy.flatten().

Example

In this example, we are flattening a nested list of population numbers using NumPy.

# Importing numpy
import numpy as np

# Creating a nested list of populations
nested_populations = [[8419600, 8982000], [2148000, 13929286], [5312163, 3331420]]

# Converting to numpy array and flattening
flat_array = np.array(nested_populations).flatten()

# Converting back to list
flat_list = flat_array.tolist()

# Displaying the flattened list
print(flat_list)

The output of the above code would be:

[8419600, 8982000, 2148000, 13929286, 5312163, 3331420]

Exercise

Select the correct option to complete each statement about flattening nested lists in Python.

1. Given nested = [[1, 2], [3, 4]], which method will flatten the list to [1, 2, 3, 4]? --Select-- [item for sublist in nested for item in sublist] nested.flatten() sum(nested)
2. What is the output of list(itertools.chain.from_iterable([[1, 2], [3, 4]]))? --Select-- [1, 2, 3, 4] [[1, 2], [3, 4]] [1, [2], 3, [4]]
3. Which module provides the chain.from_iterable() method used for flattening? --Select-- itertools collections functools