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']

# Modifies the original list
tools.append('Kubernetes')

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

O(1) Complexity

Appending to a list is an O(1) operation (constant time). Regardless of list size, adding one item to the end remains extremely fast.

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]

# Creates a brand new list
result = list_a + list_b

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

Performance Bottleneck

The + operator has O(n + m) complexity. Every use copies all elements into a new list. Avoid list = list + [item] inside loops because performance degrades as the list grows.

Performance Comparison

# ❌ Slow: O(n²) overall
result = []
for i in range(10000):
    result = result + [i]  # Creates new list each iteration

# ✅ Fast: O(n) overall
result = []
for i in range(10000):
    result.append(i)  # Modifies in place

Key Differences

Feature	.append()	+ Operator
Behavior	In-place modification	Creates new object
Return Value	None	The new list
Use Case	Building lists in loops	Joining distinct lists
Speed	🚀 O(1) constant	🐢 O(n+m) linear
Original List	Modified	Unchanged

The .extend() Alternative

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

tools = ['Git', 'Docker']

# extend: adds each element individually
tools.extend(['Kubernetes', 'Terraform'])
print(tools)  # ['Git', 'Docker', 'Kubernetes', 'Terraform']

# append: adds the list as a single nested element
tools.append(['AWS', 'GCP'])
print(tools)  # ['Git', 'Docker', 'Kubernetes', 'Terraform', ['AWS', 'GCP']]

tip

Use .extend() to add multiple items in-place. Use .append() for single items. Use + when you need a new list without modifying the originals.

Quick Reference

Goal	Method	Example
Add one item	.append(x)	lst.append(5)
Add multiple items	.extend([...])	lst.extend([5, 6])
Join two lists (new)	+	new = a + b
Join two lists (in-place)	.extend()	a.extend(b)

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.