Python Lists (List)

In this article, we will learn everything about Python lists; how to create them, how to slice them, how to add or delete elements from them, etc.

Python provides a series of compound data types that are usually called sequences. List is one of the most frequently used and widely used data types in Python.

How to create a list (List)?

In Python programming, a list is created by placing all items (elements) inside square brackets [] and separating them with commas.

It can have any number of items, and they can have different types (integers, floating-point numbers, strings, etc.).

# Empty list
my_list = []
# Integer list
my_list = [1,　2,　3]
# List with mixed data types
my_list = [1, "Hello",　3.4]

Similarly, a list can even have another list as an item. This is called a nested list.

# Nested list
my_list = ["mouse", [8,　4,　6], ['a']]

How to access elements in a list?

We can access the elements of a list in many ways.

list index

We can use the index operator [] to access items in a list. The index starts from 0. Therefore, it contains5of elements' list index ranges from 0 to4.

Attempt to access other elements; otherwise, an IndexError will be raised. The index must be an integer. We cannot use float or other types, which will cause a TypeError.

Use nested indexing to access nested lists.

my_list = ['p','r','o','b','e']
# Output: p
print(my_list[0])
# Output: o
print(my_list[2]
# Output: e
print(my_list[4]
# Error! Only integers can be used for indexing
# my_list[4].0]
# Nested list
n_list = ["Happy", [2,0,1,5]]
# Nested indexing
# Output: a
print(n_list[0][1]　　　　
# Output:　5
print(n_list[1]3]

negative index

Python allows negative indexing on sequences. The index-1represents the last item,-2represents the second-to-last item, and so on.

my_list = ['p','r','o','b','e']
# Output: e
print(my_list[-1]
# Output: p
print(my_list[-5]

How to slice a list in Python?

We can use the slicing operator (colon :) to access a series of items in a list.

my_list = ['p','r','o','g','r','a','m','i','z']
# From the3element to the5an element
print(my_list[2:5]
# From the start to the4an element
print(my_list[:-5]
# The6elements to the end
print(my_list[5:])
# Elements from start to end
print(my_list[:])

By considering the elements between the following indices, a slice can get the best visual effect. If we want to access a range, we need two indices to split that part from the list.

How to change or add elements to a list?

List is mutable, which means their elements can be changed without being likestringortuple.

We can use the assignment operator (=) to change a single item or a series of items.

# Value list
odd = [2,　4,　6,　8]
# Change the first element　　　　
odd[0] =　1　　　　　　　　　　　　
# Output: [1,　4,　6,　8]
print(odd)
# Change the2to the4item
odd[1:4] = [3,　5,　7]　　
# Output: [1,　3,　5,　7]
print(odd)

We can use the append() method to add an item to the list, or use the extend() method to add multiple items to the list.

odd = [1,　3,　5]
odd.append(7)
# Output: [1,　3,　5,　7]
print(odd)
odd.extend([9,　11,　13]
# Output: [1,　3,　5,　7,　9,　11,　13]
print(odd)

We can also use+Operator to combine two lists. This is also known as concatenation.

*Operator to repeat the list a given number of times.

odd = [1,　3,　5]
# Output: [1,　3,　5,　9,　7,　5]
print(odd　+　[9,　7,　5]
# Output: ["re", "re", "re"]
print(["re"]　*　3)

In addition, we can use the insert() method to insert an item at a required position, or insert multiple items by compressing multiple items into an empty slice of the list.

odd = [1,　9]
odd.insert(1,3)
# Output: [1,　3,　9]　
print(odd)
odd[2:2] = [5,　7]
# Output: [1,　3,　5,　7,　9]
print(odd)

How to delete or remove elements from a list?

We can use the keyword del to delete one or more items from the list. It can even completely delete the list.

my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm']
# Delete one item
del my_list[2]
# Output: ['p', 'r', 'b', 'l', 'e', 'm']　　　　　
print(my_list)
# Delete multiple items
del my_list[1:5]　　
# Output: ['p', 'm']
print(my_list)
# Delete all list items
del my_list　　　　　　　
# Error: List not defined
print(my_list)

We can use the remove() method to delete a given item, or use the pop() method to delete an item at a given index.

If no index is provided, the pop() method will delete and return the last item. This helps us implement the list as a stack (a first-in-first-out data structure).

We can also use the clear() method to empty the list.

my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm']
my_list.remove('p')
# Output: ['r', 'o', 'b', 'l', 'e', 'm']
print(my_list)
# Output: 'o'
print(my_list.pop(','))1))
# Output: ['r', 'b', 'l', 'e', 'm']
print(my_list)
# Output: 'm'
print(my_list.pop())
# Output: ['r', 'b', 'l', 'e']
print(my_list)
my_list.clear()
# Output: []
print(my_list)

Finally, we can also remove items from the list by assigning an empty list to element slicing.

>>> my_list = ['p','r','o','b','l','e','m']
>>> my_list[2:3]= []
>>> my_list
['p', 'r', 'b', 'l', 'e', 'm']
>>> my_list[2:5]= []
>>> my_list
['p', 'r', 'm']

Python list methods

Below is a list of methods available for the list object in Python programming.

They are accessed in the form list.method(). Some methods have already been used.

Some examples of Python list methods:

my_list = [3,　8,　1,　6, 0,　8,　4]
# Output:　1
print(my_list.index(8))
# Output:　2
print(my_list.count(8))
my_list.sort()
# Output: [0,　1,　3,　4,　6,　8,　8]
print(my_list)
my_list.reverse()
# Output: [8,　8,　6,　4,　3,　1, 0]
print(my_list)

List comprehension: An elegant way to create new lists

List comprehensions are a concise and clear way to create new lists from existing Python lists.

List comprehensions contain an expression followed by brackets insidefor statement.

This is a list that increments each item by2of the power example.

pow2　= [2　**　x for x in range(10)
# Output: [1,　2,　4,　8,　16,　32,　64,　128,　256,　512]
print(pow2)

This code is equivalent to

pow2　= []
for x in range(10) :
　　　pow2.append(2　**　x)

List comprehensions can choose to include more for orif statementOptional if statements can filter out items from the new list. Here are some examples.

>>> pow2　= [2　**　x for x in range(10) if x >　5]
>>> pow2
[64,　128,　256,　512]
>>> odd = [x for x in range(20) if x %　2　==　1]
>>> odd
[1,　3,　5,　7,　9,　11,　13,　15,　17,　19]
>>> [x+y for x in ['Python', 'C'] for y in ['Language', 'Programming']
['Python Language', 'Python Programming', 'C Language', 'C Programming']

Other list operations in Python

List membership test

We can use the keyword 'in' to test if an item exists in a list.

my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm']
# Output: True
print('p' in my_list)
# Output: False
print('a' in my_list)
# Output: True
print('c' not in my_list)

Traverse the list

Using a for loop, we can iterate over each item in a list.

for fruit in ['apple', 'banana', 'mango']:
　　　　print("I like", fruit)