Adding data to a list after creation is a common operation in Python. The list class supports this operation using various methods:

  1. append() - Adds a single element at the end of a list.
  2. insert()  - Adds a single element at an arbitrary position in the list.
  3. extend() - Adds all the items from the specified iterable at the end of the list.

Adding a single element at the end of the list

The append method is used to add an element at the end of the list.

x The element to be added to the list

append an element to a list

#a list of strings
languages = ['Python', 'Java']

#append an element



Insert an element at an arbitrary position 

The insert() method adds an element at a specified index in the list.

insert(i, x)
i The index at which to insert the element
x The element to be inserted

The method inserts element x at index i of the list. 

insert an element in  the list

#a list of strings

nums = ['one', 'two', 'four', 'six', 'seven', 'eight']

#insert an element at index 0
nums.insert(0, 'zero')

nums.insert(3, 'three')

nums.insert(5, 'five')

nums.insert(9, 'nine')

The insert() method is efficient when we are working with small or medium size lists. When working with large lists, it can be quite slow due to how the insertion is carried on internally. 

Extending a list with an iterable

When we have an iterable object, we can add all of its elements at the end of a list using the extend() method.

iterable The iterable(list, tuple, set, range, etc) to extend the list with.

Extend a list

# a list of strings
languages = ['Python', 'Java', 'C++']

print('before: ', languages)

# a tuple of strings
more_languages = ('PHP', 'Java', 'Ruby', 'Swift')

#extend the list with tuple elements

print('after: ', languages)

List Concatenation

List concatenation is a little different from previous approaches in that it leads to a creation of a new list. We can use this operation to combine multiple lists together into one larger list.

We use the + operator to perform string concatenation.

new_list = list1 + list2 + list3

A new list is created and all the elements of the individual lists are added to the new list in the order of their appearance.

Concatenate lists

#lists of integers
list1 = [1, 2, 3]
list2 = [4, 5, 6]
list3 = [7, 8, 9]

new_list = list1 + list2 + list3