Python offers a range of compound datatypes often referred to as sequences. List is one of the most frequently used and very versatile datatype used in Python.
In Python programming, a list is created by placing all the items (elements) inside a square bracket [ ], 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 datatypes my_list = [1, "Hello", 3.4]
Also, a list can even have another list as an item. This is called nested list.
# nested list my_list = ["mouse", [8, 4, 6], ['a']]
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. Index starts from 0. So, a list having 5 elements will have index from 0 to 4.
Trying to access an element other that this will raise an
IndexError. The index must be an integer. We can't use float or other types, this will result into
Nested list are accessed using nested indexing.
my_list = ['p','r','o','b','e'] # Output: p print(my_list) # Output: o print(my_list) # Output: e print(my_list) # Error! Only integer 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) # Output: 5 print(n_list)
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.
my_list = ['p','r','o','b','e'] # Output: e print(my_list[-1]) # Output: p print(my_list[-5])
We can access a range of items in a list by using the slicing operator (colon).
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[:])
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 index that will slice that portion from the list.
We can use assignment operator (=) to change an item or a range of items.
# mistake values odd = [2, 4, 6, 8] # change the 1st item odd = 1 # Output: [1, 4, 6, 8] print(odd) # change 2nd to 4th items odd[1:4] = [3, 5, 7] # Output: [1, 3, 5, 7] print(odd)
We can add one item to a list using
append() method or add several items using
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 called concatenation.
The * operator repeats a list for the 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)
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.
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)
We can delete one or more items from a list using the keyword
del. It can even delete the list entirely.
my_list = ['p','r','o','b','l','e','m'] # delete one item del my_list # 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 entire list del my_list # Error: List not defined print(my_list)
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 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)
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']
Methods that are available with list object in Python programming are tabulated below.
They are accessed as
list.method(). Some of the methods have already been used above.
|Python List Methods|
|append() - Add an element to the end of the list|
|extend() - Add all elements of a list to the another list|
|insert() - Insert an item at the defined index|
|remove() - Removes an item from the list|
|pop() - Removes and returns an element at the given index|
|clear() - Removes all items from the list|
|index() - Returns the index of the first matched item|
|count() - Returns the count of number of items passed as an argument|
|sort() - Sort items in a list in ascending order|
|reverse() - Reverse the order of items in the list|
|copy() - Returns a shallow copy of the list|
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 is an elegant and concise way to create new list from an existing list in Python.
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)] # 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)
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']
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)
for loop we can iterate though each item in a list.
for fruit in ['apple','banana','mango']: print("I like",fruit)
Built-in functions like
sorted() etc. are commonly used with list to perform different tasks.
|all()||Return True if all elements of the list are true (or if the list is empty).|
|any()||Return True if any element of the list is true. If the list is empty, return False.|
|enumerate()||Return an enumerate object. It contains the index and value of all the items of list as a tuple.|
|len()||Return the length (the number of items) in the list.|
|list()||Convert an iterable (tuple, string, set, dictionary) to a list.|
|max()||Return the largest item in the list.|
|min()||Return the smallest item in the list|
|sorted()||Return a new sorted list (does not sort the list itself).|
|sum()||Return the sum of all elements in the list.|