Python Beginner Concepts Tutorial

# Lists - Intro to the Data Structure & Common Operations

### What are Lists¶

Lists are mutable sequence types to store a collection of items. Mutable is the ability to alter the items in place without re-creating a new list.

Python lists can be collections of items you have on your computer such as:

• browser bookmarks
• songs in a playlist
• history of videos watched on YouTube

#### Construct a List¶

All lists are assigned a name and enclosed in square brackets.

List below is assigned to the variable name math_grades and contains a collection of scores for math exams for a single student.

There are 4 items in the list below.

In :
math_grades = [88, 91, 82, 84]

In :
math_grades

Out:
[88, 91, 82, 84]

We can further verify math_grades is a list by checking its type.

In :
type(math_grades)

Out:
list

#### Change an element in a list (mutability)¶

Since lists are mutable, we can change elements in the existing list data structure.

We can change the last grade to be 100. Re-assign the item at index 3 to be 100.

In :
math_grades = 100

In :
math_grades

Out:
[88, 91, 82, 100]

#### Lists Can Hold Multiple Object Types¶

Lists can contain multiple types of objects such as:

• numeric types:
• floats (example: 3.14)
• ints (example: 3)
• sequence types:
• strings (example: "pass")
• lists (example: [1, 2, 3])
• tuples (example: (8, "pass"))
• and more!
In :
various_grades = [82, "pass", ["great", "ok", "very good"]]

In :
various_grades

Out:
[82, 'pass', ['great', 'ok', 'very good']]

#### Lists Are Ordered Sequences of Elements¶

The order of items in a list matters. Let's take a look at math_grades.

In :
math_grades

Out:
[88, 91, 82, 84]

88 is the score on the first exam, 91 on the second exam and so forth.

#### Index Items in a List¶

Index a list to retrieve a single or multiple items in a list. An index number is always an integer value and the first item in a list is index number 0.

For the list math_grades, here's the index breakdown:

index value
0 88
1 91
2 82
3 84

Retrieve the item at index 0.

In :
math_grades

Out:
88

Retrieve the item at index 1.

In :
math_grades

Out:
91`