list.extend()

list.extend(iterable)

Returns: None · Updated March 14, 2026 · List Methods

lists methods mutation

The .extend() method adds all elements from an iterable to the end of a list. It modifies the list in-place and returns None, meaning you never assign its result to a variable. This is the idiomatic way to merge two lists or add elements from any iterable (tuples, sets, strings, generators) into an existing list.

Syntax

list.extend(iterable)

Parameters

Parameter	Type	Default	Description
`iterable`	iterable	—	An iterable (list, tuple, set, string, generator, etc.) whose elements will be added to the list.

Return Value

Returns None. The list is modified in-place.

Examples

Basic usage - extending with another list

Add elements from one list to another:

fruits = ["apple", "banana"]
fruits.extend(["cherry", "date"])
print(fruits)
# ['apple', 'banana', 'cherry', 'date']

Extending with a tuple

Tuples work just like lists:

numbers = [1, 2, 3]
numbers.extend((4, 5, 6))
print(numbers)
# [1, 2, 3, 4, 5, 6]

Extending with a set

Sets are unordered but work for adding unique elements:

unique = [1, 2, 3]
unique.extend({4, 5, 6})
print(unique)
# [1, 2, 3, 4, 5, 6]

Extending with a string

Strings are iterable — extending with a string adds each character:

letters = ["a", "b"]
letters.extend("cd")
print(letters)
# ['a', 'b', 'c', 'd']

# If you want to add the whole string as one element, use .append()
letters = ["a", "b"]
letters.append("cd")
print(letters)
# ['a', 'b', 'cd']

Extending with a generator

Generators work efficiently without loading everything into memory first:

def generate_numbers(n):
    for i in range(n):
        yield i * 2

numbers = [1, 2, 3]
numbers.extend(generate_numbers(5))
print(numbers)
# [1, 2, 3, 0, 2, 4, 6, 8]

Extending with a range

The range() object is iterable and useful for adding sequences:

sequence = [0, 1, 2]
sequence.extend(range(3, 8))
print(sequence)
# [0, 1, 2, 3, 4, 5, 6, 7]

Common Patterns

Merging two lists

The most common use case is merging lists:

list1 = [1, 2, 3]
list2 = [4, 5, 6]
list1.extend(list2)
print(list1)
# [1, 2, 3, 4, 5, 6]

# Equivalent but less idiomatic:
# list1 = list1 + list2  # creates a new list

Building a list from multiple sources

Extend works with any combination of iterables:

result = []
result.extend([1, 2, 3])      # start with a list
result.extend((4, 5))         # add a tuple
result.extend({6, 7})          # add a set
result.extend("hi")           # add characters from string
result.extend(range(10, 12))   # add from range
print(result)
# [1, 2, 3, 4, 5, 6, 7, 'h', 'i', 10, 11]

Filtering and extending in one step

A useful pattern combines filtering with extension:

source = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
target = []
target.extend(x for x in source if x % 2 == 0)
print(target)
# [2, 4, 6, 8, 10]

The difference between append and extend

Understanding when to use each is crucial:

# .append() adds a single element
data = [1, 2, 3]
data.append([4, 5])
print(data)
# [1, 2, 3, [4, 5]]  — nested list

# .extend() adds each element
data = [1, 2, 3]
data.extend([4, 5])
print(data)
# [1, 2, 3, 4, 5]  — flattened

Performance Note

.extend() with a list is O(k) where k is the length of the added iterable
It’s more efficient than using += repeatedly in a loop
For very large iterables, using a generator expression avoids creating an intermediate list in memory

# Efficient: generator doesn't create intermediate list
large_list = []
large_list.extend(x for x in range(1000000))