How to Merge Lists in Python

Python offers numerous data types for storing and manipulating information. Lists, tuples, sets, and dictionaries are among the most frequently used.

• List: An unordered collection of data that can contain duplicate elements.
• Tuple: An ordered collection where the order cannot be changed.
• Dictionaries are similar to sets but organized as key-value pairs, allowing for efficient value retrieval based on keys.
• Sets: Collections of unique, unordered elements.

Lists, however, are simple ordered collections of elements, allowing for flexible additions and deletions as needed. They are particularly useful for dynamically tracking multiple elements.

In this guide, we’ll explore how to merge lists in Python 3.11, providing examples to demonstrate their functionality.

How to Run Examples from This Guide

Copy link

If you're new to Python, here’s how to run examples from this tutorial to practice list merging:

1. Open any text editor and create a file, e.g., main.py.
2. Copy the code from one of the examples into this file and save it.
3. On Windows, open the Command Prompt; on Linux/macOS, open the terminal.
4. Navigate to the directory where your file is located using the cd command, e.g.:

cd C:\Users\

5. Execute the following command to run your script:

python main.py

Or:

python3 main.py

The result of the program execution will be displayed in the console.

Method 1: The + Operator

Copy link

The + operator can be used to merge two lists in Python. It appends one list to the end of another, resulting in a new list.

a1 = [1, 12, 5, 49, 56]
a2 = [27, 36, 42]
list= a1 + a2
print(list)

Output:

[1, 12, 5, 49, 56, 27, 36, 42]

Let’s look at another example, where a Python program generates three lists with random numbers and combines them into a single list:

import random

def generate_and_combine_lists(length):
    if length <= 0:
        raise ValueError("List length must be a positive number")

    list1 = [random.randint(1, 10) for _ in range(length)]
    list2 = [random.randint(1, 100) for _ in range(length)]
    list3 = [random.randint(1, 1000) for _ in range(length)]

    try:
        combined_list = list1 + list2 + list3
        return list1, list2, list3, combined_list
    except TypeError as e:
        print(f"Error combining lists: {e}")
        return None

list_length = 5
list1, list2, list3, combined_list = generate_and_combine_lists(list_length)

if combined_list:
    print(f"List 1: {list1}")
    print(f"List 2: {list2}")
    print(f"List 3: {list3}")
    print(f"Combined List: {combined_list}")

Output:

List 1: [4, 7, 3, 2, 10]  
List 2: [43, 73, 5, 61, 39]  
List 3: [500, 315, 935, 980, 224]  
Combined List: [4, 7, 3, 2, 10, 43, 73, 5, 61, 39, 500, 315, 935, 980, 224] 

Method 2: The * Operator

Copy link

The * operator can easily combine lists in Python by unpacking the elements of collections into indexed positions.

If you have two lists, for example:

list1 = [1, 12, 5, 49, 56]
list2 = [27, 36, 42]

Using the * operator replaces the list with its individual elements at the specified index positions, effectively "unpacking" the list contents.

list1 = [1, 12, 5, 49, 56]
list2 = [27, 36, 42]
combined_list = [*list1, *list2]
print(str(combined_list))

Output:

[1, 12, 5, 49, 56, 27, 36, 42]

Below is another example where randomly generated Python lists are combined using the * operator:

import random

def generate_and_combine_lists(length):
    if length <= 0:
        raise ValueError("List length must be a positive number")

    list1 = [random.randint(1, 100) for _ in range(length)]
    list2 = [random.randint(1, 100) for _ in range(length)]
    list3 = [random.randint(1, 100) for _ in range(length)]

    return list1, list2, list3, *list1, *list2, *list3

list_length = 5
list1, list2, list3, *combined_list = generate_and_combine_lists(list_length)

print(f"List 1: {list1}")
print(f"List 2: {list2}")
print(f"List 3: {list3}")
print(f"Combined List: {combined_list}")

Output:

List 1: [10, 43, 17, 74, 99]  
List 2: [65, 91, 56, 37, 37]  
List 3: [33, 39, 87, 27, 82]  
Combined List: [10, 43, 17, 74, 99, 65, 91, 56, 37, 37, 33, 39, 87, 27, 82]  

The * operator efficiently merges the contents of list1, list2, and list3 into a single combined_list.

Method 3: Using a for Loop

Copy link

In this method, we use a for loop to iterate over the second list. Each element from the second list is added to the first list using the append() method. The result is a new list that combines the elements of both lists.

list1 = [6, 11, 32, 71, 3]
list2 = [18, 54, 42]

print("Original List 1:", str(list1))
for x in list2:
    list1.append(x)
print("Combined List:", str(list1))

Output:

Original List 1: [6, 11, 32, 71, 3]  
Combined List: [6, 11, 32, 71, 3, 18, 54, 42]  

Method 4: List Comprehension

Copy link

We can also use list comprehensions in Python to combine lists efficiently. A list comprehension is a concise way to generate a new list based on an iterable.

list1 = [5, 73, 232, 1, 8, 19]  
list2 = [84, 56, 7, 10, 20, 30]  
combined_list = [j for i in [list1, list2] for j in i]  
print("Combined List:", str(combined_list))

Output:

[5, 73, 232, 1, 8, 19, 84, 56, 7, 10, 20, 30]  

Method 5: Using the extend() Method

Copy link

The extend() method in Python iterates over the elements of the provided list and appends them to the current list, effectively merging both lists.

import random

list1 = [random.randint(10, 20) for _ in range(5)]
list2 = [random.randint(20, 50) for _ in range(3)]

print("First List:", str(list1))
list1.extend(list2)
print("Combined List:", str(list1))

Output:

First List: [19, 19, 16, 17, 16]
Combined List: [19, 19, 16, 17, 16, 47, 21, 31]

In this approach, all elements from list2 are added to list1, updating list1 directly with the combined contents.

Method 6: Using itertools.chain()

Copy link

The itertools module in Python provides various functions for working with iterators, which can be used to efficiently generate lists. It is particularly useful for generating large lists created with complex rules, as it avoids creating the entire list in memory at once, which can lead to memory overflow for very large datasets.

We can also use the itertools.chain() function from the itertools module to combine lists in Python.

import itertools

list_of_lists = [[1, 5], [3, 4], [7, 12]]
chained_list = list(itertools.chain(*list_of_lists))
print(chained_list)

Output:

[1, 5, 3, 4, 7, 12]

Let's consider a case where we generate letters and combine them into a list.

import itertools
import string

def generate_letter_range(start, stop):
    for letter in string.ascii_lowercase[start:stop]:
        yield letter

list1 = generate_letter_range(0, 3)
list2 = generate_letter_range(7, 16)

combined_list = list(itertools.chain(list1, list2))
print(combined_list)

Output:

['a', 'b', 'c', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p']

We can also combine lists of numbers using itertools.chain().

import itertools

list1 = [5, 73, 232, 1, 8]
list2 = [19, 84, 56, 7]
list3 = [10, 20, 30]

combined_list = list(itertools.chain(list1, list2, list3))
print(combined_list)

Output:

[5, 73, 232, 1, 8, 19, 84, 56, 7, 10, 20, 30]

Let's generate random letters in two lists, with one list containing 3 letters and the other containing 7, and then combine them.

import itertools
import random
import string

def generate_letter_list(num_letters):
    for i in range(num_letters):
        yield random.choice(string.ascii_letters)

num_list1 = 3
num_list2 = 7

list1 = generate_letter_list(num_list1)
list2 = generate_letter_list(num_list2)

combined_list = list(itertools.chain(list1, list2))
print(combined_list)

Output:

['d', 'e', 'O', 'M', 'q', 'i', 'N', 'V', 'd', 'C']

Conclusion

Copy link

Each of these methods for merging lists in Python has its own particularities, and the choice of which one to use depends on what you need to accomplish, the amount of data you have, and how quickly you want to get the result. Understanding these methods will help you to work more efficiently with data in your Python projects. Choose the method that suits your needs, and don't hesitate to try different approaches to get the best result!