How do make a flat list out of list of lists in Python?

A nested list is a list that contains other lists as elements. For example: [[1,2,3], [4,5,6], [7,8,9]] is a nested list with 3 sublists as its elements.

To flatten a list of lists (convert a 2D list into a 1D list), Python offers several approaches: nested for loops, list comprehension, built-in functions like itertools.chain(), or using libraries like NumPy.

In this article, we will explore these methods to flatten a list in Python.

Using Nested For Loops

The most straightforward approach is to iterate through each sublist and add its elements to a new flat list ?

def flatten_list(nested_list):
    flat_list = []
    for sublist in nested_list:
        if isinstance(sublist, list):
            for item in sublist:
                flat_list.append(item)
        else:
            flat_list.append(sublist)
    return flat_list

nested_data = [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
result = flatten_list(nested_data)

print('Original List:', nested_data)
print('Flattened List:', result)

Original List: [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
Flattened List: [11, 12, 13, 14, 15, 16, 17, 18, 19]

Using List Comprehension

List comprehension provides a more concise and Pythonic way to flatten nested lists ?

nested_data = [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
flat_list = [item for sublist in nested_data for item in sublist]

print('Original List:', nested_data)
print('Flattened List:', flat_list)

Original List: [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
Flattened List: [11, 12, 13, 14, 15, 16, 17, 18, 19]

Using itertools.chain()

The itertools.chain() function efficiently flattens iterables by chaining them together ?

import itertools

nested_data = [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
flat_list = list(itertools.chain.from_iterable(nested_data))

print('Original List:', nested_data)
print('Flattened List:', flat_list)

Original List: [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
Flattened List: [11, 12, 13, 14, 15, 16, 17, 18, 19]

Using NumPy

NumPy provides array manipulation functions that can flatten nested structures ?

import numpy as np

nested_data = [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
flat_list = list(np.concatenate(nested_data))

print('Original List:', nested_data)
print('Flattened List:', flat_list)

Original List: [[11, 12, 13], [14, 15, 16, 17], [18, 19]]
Flattened List: [11, 12, 13, 14, 15, 16, 17, 18, 19]

Comparison

Conclusion

Use list comprehension for simple cases and readable code. For large datasets or performance-critical applications, itertools.chain() is the most efficient choice. NumPy works best with numerical data in scientific computing contexts.