Understand Python List Append, Extend, + and += Operations: A Beginner Guide – Python Tutorial

By | December 14, 2019

There are several ways to to add an element to a python list, such as list.append(), list.extend(), +, += or itertools.chain(). list.append(), list.extend(), + and += are most common used in python script. What about the differences among them? In this tutorial, we will discuss this topic.

list.append()

append(self, object: Any) -> None

This function will add an object (any types) to the end of a list.

Here is an example:

l = []

#add a integer
l.append(1)

#add a string
l.append('tutorialexample.com')

#add a list
l.append(['Tutotial', 'Example'])

print(l)

The l is:

[1, 'tutorialexample.com', ['Tutotial', 'Example']]

In this example, we create an empty python list l first. Then, we will add an integer, a string and a list to the end of list l one by one.

list.extend()

extend(self, iterable: Iterable) -> None

This function will receive an iterable object, then add each element in this iterable object to the end of list one by one.

Here is an example:

l = []

#python list
l1 = [1, 'tutorialexample.com']
l.extend(l1)

#python tuple
l2 = (2, 'https://www.tutorialexample.com')
l.extend(l2)

print(l)

The python list l is:

[1, 'tutorialexample.com', 2, 'https://www.tutorialexample.com']

In this example, we create an empty python list l first. Then,we create two iterable objects, a python list l1 and a python tuple l2. Finally, we will add the elements in l1 and l2 to the end of l1 one by one.

Notice: you should notice list.append() and list.extend() will return None.

+ operation

__add__(self, value: List) -> List

Python + operation will recevie a list object, then return a new list, which is very similar to python.extend() except it will return a new python list.

We will use an example to explain it.

l = [1, 2]

#python list
l1 = [1, 'tutorialexample.com']
l_new = l + l1

print(l)
print(l_new)

The result is:

old python list l: [1, 2]

new pythn list l_new: [1, 2, 1, ‘tutorialexample.com’]

In this example, we use + operation to add l and l1, which will add the elements in l1 to the end of l one by one, then return a new python list l_new to save all elements and the l is not changed.

+= operation

__iadd__(self, value: Iterable) -> None

This function will recevie an iterable obect, which is similar to python list.extend()

Here is an example:

l = [1, 2]

#python list
l1 = [1, 'tutorialexample.com']
l += l1

print(l)

Run this python script, python l is: [1, 2, 1, ‘tutorialexample.com’], which means l += l1 is equivalent to l.extend(l1).

To summarize:

Operation Function Return Value
list.append() Add any type data to the end return None
list.extend() Add elements in an iterable  to the end one by one return None
+ like list.extend() return new python list
+= is same to list.extend() return None

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