Python offers a range of compound data types often referred to as sequences. List is one of the most frequently used and very versatile data types used in Python.
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']]
Access List Elements
There are various ways in which we can access the elements of a list.
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
Nested lists are accessed using nested indexing.
# List indexing my_list = ['p', 'r', 'o', 'b', 'e'] # Output: p print(my_list) # Output: o print(my_list) # Output: e print(my_list) # Nested List n_list = ["Happy", [2, 0, 1, 5]] # Nested indexing print(n_list) print(n_list) # Error! Only integer can be used for indexing print(my_list[4.0])
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
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:
How to slice lists in Python?
We can access a range of items in a list by using the slicing operator
# 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[:])
['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.
Add/Change List Elements
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 = 1 print(odd) # change 2nd to 4th items odd[1:4] = [3, 5, 7] print(odd)
[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
# Appending and Extending lists in Python odd = [1, 3, 5] odd.append(7) print(odd) odd.extend([9, 11, 13]) print(odd)
[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.
* 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)
[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)
[1, 3, 9] [1, 3, 5, 7, 9]
Delete/Remove List Elements
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 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)
['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.
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)
['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)
1 2 [0, 1, 3, 4, 6, 8, 8] [8, 8, 6, 4, 3, 1, 0]
List Comprehension: Elegant way to create Lists
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 statement 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)
[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
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)
True False True
Iterating Through a List
for loop we can iterate through each item in a list.
for fruit in ['apple','banana','mango']: print("I like",fruit)
I like apple I like banana I like mango