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List

Lists are sequences. It is an object that can contain other objects, whatever their type is. You can have lists of integers [1, 2, 50, 2000, …], lists of floats, lists of string or lists containing all of those types.

Create

There are two methods to create an empty list. You can either use the list() function, or you can just initialise a variable to an empty list using [] since those are list delimiters in Python.

You can also create a list that is not empty and directly indicate the objects that it contains.

Examples

🐍 Python Script
my_list = list()
We create an empty list.

🐍 Python Script
my_list = []
We create an empty list.

🐍 Python Script
my_list = [1,2,3,4,5]
We create a list with 5 integers going from 1 to 5.

🐍 Python Script
my_list = [3.5,1,"hello",[]]
We create a list containing 4 different objects: a float, an integer, a string and a new empty list.

Insert

There are three different ways to insert elements in a list.

append

We use the method append to add an element at the end of the list. We just need to give the element we want to add as a parameter.

insert

We use the method insert to add an element where we want in the list. This time, we need to give the element we want to add and where we want to add it as parameters.

extend

We use the method extend to concatenate two lists (or more) into one. You just need to apply the method on the list you want to extend (my_list1) and give the list you want to add as a parameter. You can also achieve the same result using the “+” operator.

Examples

🐍 Python Script
my_list = [1,2,3]
my_list.append(56)
print(my_list)

We add 56 at the end of the list. This code will print: [1, 2, 3, 56].

🐍 Python Script
my_list = ['a','b','d','e']
my_list.insert(2, 'c')
print(my_list)

We insert c at the index 2. This code will print: ['a', 'b', 'c', 'd', 'e'].

🐍 Python Script
my_list1 = [3,4,5]
my_list2 = [8,9,10]
my_list1.extend(my_list2)

We extend my_list1 by inserting my_list2 at the end of it. This code will print: [3, 4, 5, 8, 9, 10].

The below code would give us the same result:

🐍 Python Script
my_list1 = [3,4,5]
my_list1 = my_list1 + my_list2

Delete

There are two different ways to remove an element from a list.

del

When using the del key word, you need to indicate as a parameter the index of the element you want to delete.

remove

When using the remove method, you need to indicate as a parameter the element you want to delete.

Examples

🐍 Python Script
my_list = [-5,-2,1,4,7,10]
del my_list[0]
print(my_list) # will print [-2, 1, 4, 7, 10]
del my_list[2] 
print(my_list) # will print [-2, 1, 7, 10]

We delete the first element of the list and then the third element of the list.

🐍 Python Script
my_list = [31,32,33,34,35]
my_list.remove(32)
print(my_list) # will print [31, 33, 34, 35]

Select and Modify

Let’s now see how we can access the different elements of a list.

You access elements from a list the same way you access characters from a string: you indicate between [] the index of the element you want. The big difference is that an element in a string can’t be changed while you can change an element in a list.

To select part of a list, you can use the same method we learned to select part of a String.

📋 Output
>>> my_list = ['c','f','m']
>>> my_list[0]
'c'
>>> my_list[2]
'm'
>>> my_list[1] = 'Z'
>>> my_list
['c', 'Z', 'm']
>>> 
>>> my_list = ["a", 3, "coucou", 7, 10]
>>> my_list[1:3]
[3, 'coucou']
>>> my_list[2:4]
['coucou', 7]
>>> my_list[:3]
['a', 3, 'coucou']
>>> my_list[2:]
['coucou', 7, 10]
>>>

Summary

  • You can directly access an element from a list using the index method: my_list[index] and you can modify it.

  • It is possible to select part of a list by writing: my_list[start_index:end_index]

    • The end_index is not included in the selection.
    • If you don’t specify the start_index, Python will assume that it is 0.
    • If you don’t specify the end_index, Python will assume that it is the last index of the list.

for and in keywords

When we want to go through each element of the list, the easiest way to do it is to use a for loop.

For example, I would like to print each element from my list on a separate row. I can use a for loop with the in keyword to go through my list and do this:

🐍 Python Script
people = ["Gertrude", "Marc", "Pierre", "Gaetan", "Jacques", "Sancho"]
for elem in people:
    print(elem)

The value of the control variable elem will be first Gertrude, then Marc, etc,… So if I print each time elem, I will print all the elements from the list.

📋 Output
Gertrude
Marc
Pierre
Gaetan
Jacques
Sancho

Another interesting use of the in keyword is when you want to check if a specific element is in your list.

For example, let's check if Thomas is already in my list of people. If he is not, let's add him:

🐍 Python Script
people = ["Gertrude", "Marc", "Pierre", "Gaetan", "Jacques", "Sancho"]
if "Thomas" in people:
    print("This name is already in the list.")
else:
    print("This name is not yet in the list, let's add it.")
    people.append("Thomas")