How to Add Elements to Lists in Python using Plus Operator and append() Method

Choosing the right way to grow your lists impacts both readability and performance. While the + operator and .append() method both increase collection size, they behave differently in memory and serve distinct purposes.

Using .append(): In-Place Modification

The .append() method modifies the original list directly. It's the standard way to build lists dynamically, especially in loops.

tools = ['Git', 'Docker']

tools.append('Kubernetes')

print(tools)  # ['Git', 'Docker', 'Kubernetes']

Using the + Operator: Concatenation

The + operator joins two sequences to create an entirely new object. It doesn't modify the originals, making it ideal when immutability matters.

list_a = [1, 2]
list_b = [3, 4]

result = list_a + list_b

print(list_a)   # Still [1, 2]
print(result)   # [1, 2, 3, 4]

Performance Comparison

result = []
for i in range(10000):
    result = result + [i]  # Creates new list each iteration

result = []
for i in range(10000):
    result.append(i)  # Modifies in place

Key Differences

The .extend() Alternative

Add multiple elements in-place without creating a new list:

tools = ['Git', 'Docker']

tools.extend(['Kubernetes', 'Terraform'])
print(tools)  # ['Git', 'Docker', 'Kubernetes', 'Terraform']

tools.append(['AWS', 'GCP'])
print(tools)  # ['Git', 'Docker', 'Kubernetes', 'Terraform', ['AWS', 'GCP']]

Quick Reference

Summary

Use .append() for building lists dynamically because it's fast and memory-efficient. Use + when you need a new list without modifying the originals. For adding multiple elements in-place, use .extend(). Avoid list = list + [item] in loops as it creates unnecessary copies and degrades performance.