Python List

How to create a list?

In Python programming, a list is created by placing all the items (elements) inside square brackets [], separated by commas.

It can have any number of items and they may be of different types (integer, float, string etc.).

# empty list
my_list = []
# list of integers
my_list = [1, 2, 3]
# list with mixed data types
my_list = [1, "Hello", 3.4]

A list can also 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 from a list?

There are various ways in which we can access the elements of a list.

List Index

We can use the index operator [] to access an item in a list. In Python, indices start at 0. So, a list having 5 elements will have an index from 0 to 4.

Trying to access indexes other than these will raise an IndexError. The index must be an integer. We can't use float or other types, this will result in TypeError.

Nested lists are accessed using nested indexing.

# List indexing
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])
# Nested List
n_list = ["Happy", [2, 0, 1, 5]]
# Nested indexing
print(n_list[0][1])
print(n_list[1][3])
# Error! Only integer can be used for indexing
print(my_list[4.0])

Output

p
o
e
a
5
Traceback (most recent call last):
  File "<string>", line 21, in <module>
TypeError: list indices must be integers or slices, not float

Negative indexing

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

# Negative indexing in lists
my_list = ['p','r','o','b','e']
print(my_list[-1])
print(my_list[-5])

When we run the above program, we will get the following output:

e
p

How to slice lists in Python?

We can access a range of items in a list by using the slicing operator :(colon).

# List slicing in Python

my_list = ['p','r','o','g','r','a','m','i','z']

# elements 3rd to 5th
print(my_list[2:5])

# elements beginning to 4th
print(my_list[:-5])

# elements 6th to end
print(my_list[5:])

# elements beginning to end
print(my_list[:])

Output

['o', 'g', 'r']
['p', 'r', 'o', 'g']
['a', 'm', 'i', 'z']
['p', 'r', 'o', 'g', 'r', 'a', 'm', 'i', 'z']

Slicing can be best visualized by considering the index to be between the elements as shown below. So if we want to access a range, we need two indices that will slice that portion from the list.

How to change or add elements to a list?

Lists are mutable, meaning their elements can be changed unlike string or tuple.

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

# Correcting mistake values in a list
odd = [2, 4, 6, 8]

# change the 1st item    
odd[0] = 1            

print(odd)

# change 2nd to 4th items
odd[1:4] = [3, 5, 7]  

print(odd)

Output

[1, 4, 6, 8]
[1, 3, 5, 7]

We can add one item to a list using the append() method or add several items using extend() method.

# Appending and Extending lists in Python
odd = [1, 3, 5]

odd.append(7)

print(odd)

odd.extend([9, 11, 13])

print(odd)

Output

[1, 3, 5, 7]
[1, 3, 5, 7, 9, 11, 13]

We can also use + operator to combine two lists. This is also called concatenation.

The * operator repeats a list for the given number of times.

# Concatenating and repeating lists
odd = [1, 3, 5]

print(odd + [9, 7, 5])

print(["re"] * 3)

Output

[1, 3, 5, 9, 7, 5]
['re', 're', 're']

Furthermore, we can insert one item at a desired location by using the method insert() or insert multiple items by squeezing it into an empty slice of a list.

# Demonstration of list insert() method
odd = [1, 9]
odd.insert(1,3)

print(odd)

odd[2:2] = [5, 7]

print(odd)

Output

[1, 3, 9]
[1, 3, 5, 7, 9]

How to delete or remove elements from a list?

We can delete one or more items from a list using the keyword del. It can even delete the list entirely.

# Deleting list items
my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm']

# delete one item
del my_list[2]

print(my_list)

# delete multiple items
del my_list[1:5]

print(my_list)

# delete entire list
del my_list

# Error: List not defined
print(my_list)

Output

['p', 'r', 'b', 'l', 'e', 'm']
['p', 'm']
Traceback (most recent call last):
  File "<string>", line 18, in <module>
NameError: name 'my_list' is not defined

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

The pop() method removes and returns the last item if the index is not provided. This helps us implement lists as stacks (first in, last out data structure).

We can also use the clear() method to empty a 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)

Output

['r', 'o', 'b', 'l', 'e', 'm']
o
['r', 'b', 'l', 'e', 'm']
m
['r', 'b', 'l', 'e']
[]

Finally, we can also delete items in a list by assigning an empty list to a slice of elements.

>>> 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

Methods that are available with list objects in Python programming are tabulated below.

They are accessed as list.method(). Some of the methods have already been used above.

Some examples of Python list methods:

# 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)

Output

1
2
[0, 1, 3, 4, 6, 8, 8]
[8, 8, 6, 4, 3, 1, 0]

List Comprehension: Elegant way to create new List

List comprehension is an elegant and concise way to create a new list from an existing list in Python.

A list comprehension consists of an expression followed by for statements inside square brackets.

Here is an example to make a list with each item being increasing power of 2.

pow2 = [2 ** x for x in range(10)]
print(pow2)

Output

[1, 2, 4, 8, 16, 32, 64, 128, 256, 512]

This code is equivalent to:

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

A list comprehension can optionally contain more for or if statements . An optional if statement can filter out items for 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 test if an item exists in a list or not, using the keyword in.

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)

Output

True
False
True

Iterating Through a List

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

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

Output

I like apple
I like banana
I like mango