7.1.2. Manipulating Elements

Data stored in a list can be modified after list has been initially defined. Let us have a look at some ways of doing that.

Replacing Single Element

Elements in a list can be replaced using the indexes.

CodeOutput

fruits = ['apple', 'banana', 'peach', 'strawberry']
print('before:', fruits)

fruits[1] = 'grapes'
print('after:', fruits)

before: ['apple', 'banana', 'peach', 'strawberry']
after: ['grapes', 'banana', 'peach', 'strawberry']

Note that it isn't possible to add additional elements using indexes:

CodeOutput

fruits = ['apple', 'banana', 'peach', 'strawberry']
print('before:', fruits)

fruits[4] = 'grapes'
print('after:', fruits)

before: ['apple', 'banana', 'peach', 'strawberry']
Traceback (most recent call last):
  File "C:\XGuides\python\test.py", line 4, in <module>
    fruits[4] = 'grapes'
    ~~~~~~^^^
IndexError: list assignment index out of range

Since list only has four elements, maximum index is 3 and we're trying to assign to 4th index which isn't allowed.

Replacing Multiple Elements

It is also possible to replace multiple elements using slices:

CodeOutput

fruits = ['apple', 'banana', 'peach', 'strawberry', 'mango', 'grapes']
print('before:', fruits)

fruits[1:4] = ['BANANA', 'PEACH', 'STRAWBERRY']
print('after:', fruits)

before: ['apple', 'banana', 'peach', 'strawberry']
after: ['apple', 'BANANA', 'PEACH', 'STRAWBERRY', 'mango', 'grapes']

Again, the upper bound is exclusive in this case so in order to replace elements from index 1 to index 3, we have to provide 1:4.

Note however that it isn't necessarily important for list being replaced to be of same size as provided index range. Additional elements are adjusted automatically:

CodeOutput

fruits = ['apple', 'banana', 'peach', 'strawberry', 'mango', 'grapes']
print('before:', fruits)

fruits[1:4] = ['BANANA', 'PEACH', 'STRAWBERRY', 'WATERMELON']
print('after:', fruits)

before: ['apple', 'banana', 'peach', 'strawberry']
after: ['apple', 'BANANA', 'PEACH', 'STRAWBERRY', 'WATERMELON', 'mango', 'grapes']

In this case, WATERMELON was the additional element.

Adding Elements (+ operator)

Elements can be added to the list in various ways.

First way is by concatenating lists. Two lists can be added such that the elements of second list (the one on right of + operator) are appended to the first list.

CodeOutput

fruits = ['apple', 'banana', 'peach']
new_fruits = fruits + ['strawberry', 'mango']

print(new_fruits)

['apple', 'banana', 'peach', 'strawberry', 'mango']

A new list is created everytime + is used between two lists.

Adding Elements (methods)

Other ways of adding elements is using the methods provided by list data type: list.append(), list.extend() and list.insert().

list.extend()

list.extend() is essentially same as using the + operator.

CodeOutput

fruits = ['apple', 'banana', 'peach']
fruits.extend(['strawberry', 'mango'])

print(fruits)

['apple', 'banana', 'peach', 'strawberry', 'mango']

One caveat about .extend() is that passed argument isn't necessarily supposed to be a list. It could be any iterable, even a string too. This isn't the case in using + operator.

CodeOutput

fruits = ['apple', 'banana', 'peach']
fruits.extend('mango')

print(fruits)

['apple', 'banana', 'peach', 'm', 'a', 'n', 'g', 'o']

Another thing to note is that + creates a new list with updated elements while .extend() updates the old list instead of creating new one. This is called in-place update operation.

list.append()

list.append() on the other hand is used to add singular elements to the end of list.

CodeOutput

fruits = ['apple', 'banana', 'peach']

fruits.append('mango')
print('appended mango:', fruits)

fruits.append('strawberry')
print('appended strawberry:', fruits)

appended mango: ['apple', 'banana', 'peach', 'mango']
appended strawberry: ['apple', 'banana', 'peach', 'mango', 'strawberry']

list.insert()

Finally, we have list.insert() method which is used to insert elements at a specific index.

CodeOutput

fruits = ['apple', 'banana', 'peach']

fruits.insert(0, 'mango')
print('insert mango at start:', fruits)

fruits.insert(2, 'strawberry')
print('insert strawberry at index 2:', fruits)

insert mango at start: ['mango', 'apple', 'banana', 'peach']
insert strawberry at index 2: ['mango', 'apple', 'strawberry', 'banana', 'peach']

Removing Elements (del)

List elements can be removed using their indexes using the del keyword.

CodeOutput

fruits = ['apple', 'banana', 'peach']
print('before:', fruits)

del fruits[1]
print('after:', fruits)

before: ['apple', 'banana', 'peach']
after: ['apple', 'peach']

Note

The del keyword is used to delete any object in Python. If we use del on a variable, that variable is deleted.

a = 1
del a
print(a)  # NameError: 'a' is not defined

Removing Elements (methods)

The list data type provides many methods that can be used to remove elements from a list.

list.pop()

list.pop() works in a similar way as the del operator: it removes the element using its index. The only difference is that the removed element is also returned by this method.

CodeOutput

fruits = ['apple', 'banana', 'peach']
print('before:', fruits)

removed = fruits.pop(1)
print(f'after removing {removed}:', fruits)

before: ['apple', 'banana', 'peach']
after removing banana: ['apple', 'peach']

fruits.pop(1) returned the removed element, 'banana'.

If no argument is passed in list.pop(), the last element of the list (from right hand side) is removed:

CodeOutput

fruits = ['apple', 'banana', 'peach']
print('before:', fruits)

removed = fruits.pop()
print(f'after removing {removed}:', fruits)

before: ['apple', 'banana', 'peach']
after removing peach: ['apple', 'banana']

list.remove()

list.remove() takes any value as parameter and removes that value from the list:

CodeOutput

fruits = ['apple', 'banana', 'peach']
print('before:', fruits)

fruits.remove('banana')
print(f'after:', fruits)

before: ['apple', 'banana', 'peach']
after: ['apple', 'peach']

Worth noting that if the given value appears more than once in the list, only the first occurence is removed:

CodeOutput

fruits = ['apple', 'banana', 'peach', 'banana']
print('before:', fruits)

fruits.remove('banana')
print(f'after:', fruits)

before: ['apple', 'banana', 'peach', 'banana']
after: ['apple', 'peach', 'banana']

In case the passed value is not in the list, a ValueError is raised.

list.clear()

list.clear() is a rather nuclear option. This method deletes all elements of the list and makes it empty.

CodeOutput

fruits = ['apple', 'banana', 'peach']
print('before:', fruits)

fruits.clear()
print(f'after:', fruits)

before: ['apple', 'banana', 'peach', 'banana']
after: []