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Python

Flatten a List of Lists in Python

By Filip on 10/05/2024

Learn how to efficiently flatten a list of lists in Python using techniques like list comprehension, the itertools library, and more.

Flatten a List of Lists in Python

Table of Contents

Introduction

In Python, you might encounter situations where you have a list containing other lists, and you need to combine all the elements into a single, flat list. This operation is called "flattening" a list of lists. Let's explore different methods to achieve this.

Step-by-Step Guide

Let's say you have a list of lists like this:

list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]

You want to "flatten" this into a single list containing all the elements: [1, 2, 3, 4, 5, 6, 7, 8]. Here's how you can do it:

1. Using Nested Loops:

flat_list = []
for sublist in list_of_lists:
  for item in sublist:
    flat_list.append(item)
  • We start with an empty list called flat_list.
  • The outer loop iterates through each sublist in list_of_lists.
  • The inner loop iterates through each item in the current sublist.
  • For each item, we use flat_list.append(item) to add it to our flat_list.

2. Using List Comprehension:

flat_list = [item for sublist in list_of_lists for item in sublist]
  • This achieves the same result as the nested loops but in a more compact way.
  • It reads almost like English: "Create a list of items for each sublist in list_of_lists and for each item in sublist".

3. Using sum():

flat_list = sum(list_of_lists, [])
  • This is a concise way to flatten a list of lists.
  • sum(iterable, start) normally adds up numbers, but we provide an empty list [] as the start value.
  • It effectively concatenates all the sublists together.

4. Using itertools.chain(*list_of_lists):

import itertools
flat_list = list(itertools.chain(*list_of_lists))
  • itertools.chain(*list_of_lists) creates an iterator that treats consecutive sequences as a single sequence.
  • The * unpacks the list_of_lists so that chain sees each sublist as a separate argument.
  • We use list() to convert the resulting iterator into a list.

These are some of the most common ways to flatten a list of lists in Python. Choose the method that you find most readable and efficient for your specific use case.

Code Example

This Python code demonstrates four different ways to flatten a list of lists: using nested loops, list comprehension, the sum() function, and the itertools.chain() function. Each method is illustrated with a clear example and its output is printed to the console for comparison.

list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]

# 1. Using Nested Loops:
flat_list_loops = []
for sublist in list_of_lists:
    for item in sublist:
        flat_list_loops.append(item)

# 2. Using List Comprehension:
flat_list_comprehension = [item for sublist in list_of_lists for item in sublist]

# 3. Using sum():
flat_list_sum = sum(list_of_lists, [])

# 4. Using itertools.chain(*list_of_lists):
import itertools
flat_list_itertools = list(itertools.chain(*list_of_lists))

print("Original list of lists:", list_of_lists)
print("Flattened using nested loops:", flat_list_loops)
print("Flattened using list comprehension:", flat_list_comprehension)
print("Flattened using sum():", flat_list_sum)
print("Flattened using itertools.chain():", flat_list_itertools)

This code demonstrates all four methods of flattening a list of lists, providing clear output to compare the results.

Additional Notes

  • Choosing the right method:
    • Readability: For simple cases, list comprehension often provides the most concise and readable solution.
    • Performance: itertools.chain(*list_of_lists) and sum(list_of_lists, []) are generally faster, especially for large lists. However, always profile your code to be sure.
    • Nested Lists: The provided methods work for one level of nesting. For deeply nested lists, you might need recursion or consider libraries like numpy for efficient flattening.
  • Important Considerations:
    • Data Types: These methods work for lists containing any data type.
    • Empty Sublists: All methods handle empty sublists gracefully.
    • Modifying Original List: None of these methods modify the original list_of_lists. They create a new flattened list.
  • Beyond the Basics:
    • functools.reduce: This function can also be used for flattening, though it might be less readable than other methods.
    • Numpy: If you're working with numerical data in arrays, numpy.flatten() provides a very efficient way to flatten arrays.

Remember that the best approach depends on the specific context of your code and the size of your data.

Summary

This article provides four methods to flatten a list of lists in Python, transforming a structure like [[1, 2, 3], [4, 5], [6, 7, 8]] into [1, 2, 3, 4, 5, 6, 7, 8].

| Method | Description

Conclusion

Flattening a list of lists is a common operation in Python, and understanding the different methods available allows you to choose the most efficient and readable solution for your specific needs. Whether you prioritize conciseness, performance, or compatibility with different data structures, Python offers a flexible toolkit for working with nested lists effectively.

References

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