PythonArraysBeginner

list.append()

Adds one item to the end of a list (mutates the list).

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my_list.append(item)
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What it does

Overview

Adds one item to the end of a list (mutates the list).

The append() method is used to add a single object to the end of a list. It is a fundamental operation that modifies the list object in-place and returns None to the caller. Performance-wise, appending is highly efficient with an amortized time complexity of O(1). This is because Python lists are implemented as dynamic arrays that over-allocate memory; reallocations (O(n)) occur only when the pre-allocated capacity is reached. A common behavior to note is that append() does not create a new list; it directly alters the internal buffer of the existing list. When an iterable such as a list, tuple, or dictionary is passed to append(), the entire collection is added as one single element, resulting in a nested structure rather than a flat extension of the original list.

Quick reference

Syntax

my_list.append(item)

Inputs

Parameters

itemAny · Item to add to the list.

See it in practice

Examples

1

Basic Appending

fruits = ['apple', 'orange']
fruits.append('banana')
print(fruits)
Output:
['apple', 'orange', 'banana']

Adds a string element to the end of the list.

2

Appending a List (Nesting)

data = [1, 2, 3]
data.append([4, 5])
print(data)
Output:
[1, 2, 3, [4, 5]]

Demonstrates that appending a list results in a nested list structure, not a flattened one.

3

Using append in a Loop

squares = []
for i in range(5):
    squares.append(i * i)
print(squares)
Output:
[0, 1, 4, 9, 16]

A common pattern for dynamically building a list from an iterative process.

Debug faster

Common Errors

1

LogicError

Cause: Attempting to assign the result of append() to a variable, which results in None because the method operates in-place.

Fix: Call append() as a standalone statement and continue using the original list variable.

my_list = [1, 2]
new_list = my_list.append(3) # new_list is None
2

TypeError

Cause: Passing multiple arguments to the append() method. It accepts exactly one argument.

Fix: Use extend() to add multiple elements or wrap the elements in a single container like a tuple.

items = [1, 2]
items.append(3, 4) # Throws TypeError
3

LogicError

Cause: Confusing append() with extend() when the goal is to merge two lists.

Fix: Use extend() if you want to flatten the elements into the list, or the '+' operator for concatenation.

a = [1]
a.append([2, 3]) # Result is [1, [2, 3]], not [1, 2, 3]

Runtime support

Compatibility

Python 3.8+

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Adds one item to the end of a list (mutates the list).

item: Item to add to the list.

LogicError: Call append() as a standalone statement and continue using the original list variable. TypeError: Use extend() to add multiple elements or wrap the elements in a single container like a tuple. LogicError: Use extend() if you want to flatten the elements into the list, or the '+' operator for concatenation.