Python Lists (With Examples)

Python offers a variety of data types. List is one of the most frequently used data types in Python.

Topics Covered

1. Characteristics of Python Lists
2. How to create a list in Python?
3. Access elements of a list
4. Slicing of Lists
5. Mixing positive and negative indices for slicing
6. Using steps in slicing
7. Reversing a list
8. Changing elements of a list
9. Adding elements to a list
10. Deleting elements from a list
11. Common Python list Methods
12. Python List Exercises

Characteristics of Python Lists

1. Python lists can contain any data type of your choice, e.g., String, Integers, Python objects etc.
2. Lists can also contain lists. They are called Nested Lists.
3. Lists are mutable. This means, you can alter the contents of a list anytime during the program.
4. The sequence in which data is stored in a list is preserved.

The above characteristics make Python lists very powerful and versatile.

How to create a list in Python?

To create an empty list, you can directly call the list() constructor. You can also declare an empty list with empty square brackets [].

Code

my_list = list()
print(my_list)

my_second_list = []
print(my_second_list)

Output

[]
[]

To create a non-empty list, you can place all the items within the square brackets [].

Code

my_list = [1, 2, 3]
print(my_list)

another_list = ["Pylenin", 1, "loves", "Python"]
print(another_list)

my_nested_list = ["Pylenin", [1, 2, 3]]
print(my_nested_list)

Output

[1, 2, 3]
["Pylenin", 1, "loves", "Python"]
["Pylenin", [1, 2, 3]]

The nested lists are also called Multi dimensional lists.

Access elements of a list

The process for accessing elements from a list, work similar to strings.

You can access individual elements of a list using indexing and a range of elements using slicing. In Python lists, index starts from 0 and keeps increasing by 1 with every element in the list. This is called Positive Indexing(or indexing from the beginning). If there are n elements in a list, the 1st element will have index of 0 and the last element will have index n-1.

Let’s look at the image below for better understanding.

Python also allows negative indexing for lists(indexing from the end). If there are n elements in a list, the last element will have index of -1 and the first element will have index -n.

Let’s look at the image below for better understanding.

Code

my_list = [1, 2, 3, 4, 5]

# first character
print('my_list[0] =', my_list[0])

# last character
print('my_list[-1] =', my_list[-1])

# second character
print('my_list[1] =', my_list[1])

# second last character
print('my_list[-2] =', my_list[-2])

Output

my_list[0] = 1
my_list[-1] = 5
my_list[1] = 2
my_list[-2] = 4

If you try to access an element out of range, it will raise an IndexError.

Also, the index must be an integer. You can’t use floats or other types for indexing. This will result in TypeError.

Code/Output

my_list = [1, 2, 3, 4, 5]

print(my_list[10])
>>> IndexError: list index out of range

print(my_list[3.5])
>>> TypeError: list indices must be integers or slices, not float

Slicing of Lists

To access a range of elements from a Python list, you need to use slicing.

Basically, you have to specify the starting index and the ending index, separated by a colon(:), to return a range of elements from the list.

Slicing Lists with Positive Index

Remember - The last index is not included.

Let’s say you want to access the 1st and the 2nd index from a list. In that case, your end index will be 3.

Code

my_list = [1, 2, 3, 4, 5]

print(my_list[1:3])

Output

[2, 3]

If you leave out the start index, the range will start at the first element.

Code

my_list = [1, 2, 3, 4, 5]

# leave out the first index
print(my_list[:3])

Output

[1, 2, 3]

If you leave out the end index, the range will go to the end.

Code

my_list = [1, 2, 3, 4, 5]

# leave out the end index
print(my_list[1:])

Output

[2, 3, 4, 5]

So, if you leave out both the start and end index, you will get a replica of the original list.

Code

my_list = [1, 2, 3, 4, 5]

# leave out the start and end index
print(my_list[:])

Output

[1, 2, 3, 4, 5]

Slicing Lists with Negative Index

You can also use negative index for slicing. Just use the negative index counterpart of your positive index to slice a list.

Code

my_list = [1, 2, 3, 4, 5]

# Slicing with Positive Index
print(my_list[1:3])
# Slicing with Negative Index
print(my_list[-4:-2])

Output

[2, 3]
[2, 3]

Mixing positive and negative indices for slicing

You can also mix positive and negative indices while slicing a list.

Code

my_list = [1, 2, 3, 4, 5]

print(my_list[1:-1])
print(my_list[-3:5])

Output

[2, 3, 4]
[3, 4, 5]

Let’s understand the above example using the below image.

Using steps in slicing

You can also include steps while slicing your list. The step allows you to take every nth element while slicing.

Syntax

list[start:stop:step]

Code

my_list = [1, 2, 3, 4, 5]

# Get every second element
# from beginning to end
print(my_list[::2])

# Get every second element
# starting at index 1 till end
print(my_list[1::2])

# Get every second element
# starting at index 1 upto -2 index
print(my_list[1:-2:2])

Output

[1, 3, 5]
[2, 4]
[2]

Reversing a list

Negative step changes the slice to be built from the tail of the list. So, it goes from the last element to the first element. This way, we get a reversed list.

Code

my_list = [1, 2, 3, 4, 5]

print(my_list[::-1])

Output

[5, 4, 3, 2, 1]

When using a negative step, the start and stop values have to be provided from right to left (usually it is from left to right).

Code

my_list = [1, 2, 3, 4, 5]

# Reverse the list
# without the last element
print(my_list[-2::-1])

# Reverse the list
# without the first and last element
print(my_list[-2:0:-1])

Output

[4, 3, 2, 1]
[4, 3, 2]

Changing elements of a list

Since lists are mutable, you can easily update or change the contents of a list. You can either change one element or change a range of elements of the list.

Code

my_list = [1, 2, 3, 4, 5]

# change the 1st element
my_list[0] = "Pylenin"
print(my_list)

# change 2nd to 5th element
my_list[1:4] = [10, 20, 30]
print(my_list)

Output

['Pylenin', 2, 3, 4, 5]
['Pylenin', 10, 20, 30, 5]

Adding elements to a list

You can add elements to list using concatenation or by using built-in methods like append(), extend() or insert().

Adding elements to a list by concatenation

Code

my_list = [1, 2, 3, 4, 5]
new_list = ["Pylenin", "loves", "Python"]

# Using + operator
print(my_list + new_list)

# Using * operator
print(my_list*2)

Output

[1, 2, 3, 4, 5, 'Pylenin', 'loves', 'Python']
[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]

Python list append

You can add one element to a list by using the append() built-in method.

Code

my_list = [1, 2, 3, 4, 5]
my_list.append(10) print(my_list)
my_list.append([10, 20]) print(my_list) 

Output

[1, 2, 3, 4, 5, 10]
[1, 2, 3, 4, 5, 10, [10, 20]]

f you try to add more than one element with append(), it throws a TypeError.

Code

my_list = [1, 2, 3, 4, 5]

my_list.append(10, 20)
print(my_list)

Output

TypeError: list.append() takes exactly one argument (2 given)

To add multiple elements to the list, you can use extend() method.

Python list extend

Code

my_list = [1, 2, 3, 4, 5]

my_list.extend([10, 20, 30])
print(my_list)

Output

[1, 2, 3, 4, 5, 10, 20, 30]

Note:- You have to pass in all the elements in the form of a list.

Python list insert

Both append() and extend() methods, add elements to the end of a list. To add elements at a certain position (specific index) in the list, you can use the insert() method.

Code

my_list = [1, 2, 3, 4, 5]

# Insert 10 at 2nd index
my_list.insert(2, 10)
print(my_list)

# Insert 10 at 10th index
my_list.insert(10, 10)
print(my_list)

Output

[1, 2, 10, 3, 4, 5]
[1, 2, 10, 3, 4, 5, 10]

In the 2nd example, you can see that when trying to insert an element at a non-existing index, Python just inserts it as the last element of the list.

Deleting elements from a list

Python list del statement

You can delete an element of a list or an entire list using the del statement.

Code

my_list = [1, 2, 3, 4, 5]

del my_list[0]
print(my_list)

del my_list
print(my_list)

Output

[2, 3, 4, 5]
Traceback (most recent call last):
  File "C:\Users\91824\PycharmProjects\pythonProject\trial.py", line 7, in <module>
    print(my_list)
NameError: name 'my_list' is not defined

Sincemy_list was deleted, Python threw an error ame 'my_list' is not defined.

Python list clear

If you don’t want to delete the list, but just clear the list of its contents, you can use the clear() built-in method.

Code

my_list = [1, 2, 3, 4, 5]

my_list.clear()
print(my_list)

Output

[]

Python list pop

To remove an element from the list and return the removed element, use the pop() method.

Syntax of pop

list.pop(index)

index: index of the element 
       to be removed(Optional)
Returns the removed element

If index is not provided, pop() removes and returns the last element of the list.

Code

my_list = [1, 2, 3, 4, 5]

print(my_list.pop())
print(my_list)

Output

5
[1, 2, 3, 4]

Python list remove

If you are sure of the kind of content existing in the list, you can also use the remove() method to remove that particular element.

Code

my_list = [1, 2, 3, 4, 5]

my_list.remove(3)
print(my_list)

# Trying to remove a non existent element
my_list.remove(10)

Output

[1, 2, 4, 5]

Traceback (most recent call last):
  File "some_file_location", line 6, in <module>
    my_list.remove(10)
ValueError: list.remove(x): x not in list

If the element you are trying to remove doesn’t exist in the list, Python throws a ValueError.

Common Python list Methods

Python list count

The count() method in Python returns the number of occurences of a particular element in the list.

Code

my_list = [1, 2, 3, 4, 5, 1]

print(my_list.count(1))

Output

2

Python list index

The index() method returns the first matched index of a particular element in the list.

Syntax of index()

list.index(element, start, stop)

element - the element to be searched
start (optional) - start searching from this index
end (optional) - search the element upto this index (Not included)

Code

my_list = [1, 2, 3, 4, 5, 1]

print("Index of 1 from beginning is, ",my_list.index(1))

print("Index of 1 from 1st index is, ",my_list.index(1, 1))

print("Index of 1 between 1st and 5th index is, ",my_list.index(1, 1, 5))

Output

Index of 1 from beginning is,  0
Index of 1 from 1st index is,  5

Traceback (most recent call last):
  File "some_file_location", line 7, in <module>
    print("Index of 1 between 1st and 5th index is, ",my_list.index(1, 1, 5))
ValueError: 1 is not in list

Your end index is not inclusive of the search result. In the above case, the end index should be 6 to return the index of 1.

Python list len

The len() function calculates the length of any given list.

Code

my_list = [1, 2, 3, 4, 5, 1]

print(f"The length of list is {len(my_list)}")

The above code uses f-strings

Output

The length of list is 6

Python list sort

The sort() method in Python sorts the list in an ascending or descending order based on the provided key.

Syntax of sort()

list.sort(key, reverse)

key(Optional): The key on which sorting will happen.
reverse(Optional): The default value is False.
                   When set to True, it sorts in descending order.

Code

my_list = [1, 4, 3, 2, 5]

#Sort in ascending order
my_list.sort()
print(my_list)

#Sort in descending order
my_list.sort(reverse=True)
print(my_list)

Output

[1, 2, 3, 4, 5]
[5, 4, 3, 2, 1]

It also works on strings.

Code

my_list = ["a", "i", "e", "u", "o"]

#Sort in ascending order
my_list.sort()
print(my_list)

#Sort in descending order
my_list.sort(reverse=True)
print(my_list)

Output

['a', 'e', 'i', 'o', 'u']
['u', 'o', 'i', 'e', 'a']

Let’s say you have a list containing strings and you would like to sort them based on length of each string. You can use the key parameter to achieve this objective.

Code

my_list = ["Pylenin", "loves", "Python"]

#Sort in ascending order of length
my_list.sort(key=len)
print(my_list)

#Sort in descending order of length
my_list.sort(key=len, reverse=True)
print(my_list)

Output

['loves', 'Python', 'Pylenin']
['Pylenin', 'Python', 'loves']

Python list reverse

The reverse() method in Python reverses the order of the list.

Code

my_list = ["Pylenin", "loves", "Python"]

#Reverse the list
my_list.reverse()
print(my_list)

Output

['Python', 'loves', 'Pylenin']

Python list copy

The copy() method in Python returns a shallow copy of the list. It is better than using the = operator as any changes to the copied list is not reflected in the original list.

Code - Copy list using =

my_list = ["Pylenin", "loves", "Python"]
new_list = my_list

new_list.append("3")

print(new_list)
print(my_list)

Output

['Pylenin', 'loves', 'Python', '3']
['Pylenin', 'loves', 'Python', '3']

As you can see, '3' was appended to both the new_list and my_list.

To avoid this, you can use the copy() method.

Code - using copy()

my_list = ["Pylenin", "loves", "Python"]
new_list = my_list.copy()

new_list.append("3")

print(new_list)
print(my_list)

Output

['Pylenin', 'loves', 'Python', '3']
['Pylenin', 'loves', 'Python'] # old list stayed the same

Python List Exercises

1. Write a Python program to get the largest number from a list.

2. Write a Python program to remove duplicates from a list.

3. Write a Python program to build a list containing 5 integer zeros.

Solve the above questions and tweet me your answer to @pylenin on Twitter.