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.

```math_grades = [88, 91, 82, 84]
```
```math_grades
```
```[88, 91, 82, 84]
```

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

```type(math_grades)
```
```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.

```math_grades[3] = 100
```
```math_grades
```
```[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!
```various_grades = [82, "pass", ["great", "ok", "very good"]]
```
```various_grades
```
```[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`.

```math_grades
```
```[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`.

```math_grades[0]
```
```88
```

Retrieve the item at index `1`.

```math_grades[1]
```
```91
```
```
```