Python Lists
List
Lists are used to store multiple items in a single variable.
Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple, Set, and Dictionary, all with different qualities and usage.
Lists are created using square brackets:
Example
Create a List:
print[thislist]
List Items
List items are ordered, changeable, and allow duplicate values.
List items are indexed, the first item has index [0], the second item has index [1] etc.
Ordered
When we say that lists are ordered, it means that the items have a defined order, and that order will not change.
If you add new items to a list, the new items will be placed at the end of the list.
Note: There are some list methods that will change the order, but in general: the order of the items will not change.
Changeable
The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.
Allow Duplicates
Since lists are indexed, lists can have items with the same value:
Example
Lists allow duplicate values:
print[thislist]
List Length
To determine how many items a list has, use the len[] function:
Example
Print the number of items in the list:
print[len[thislist]]
List Items - Data Types
List items can be of any data type:
Example
String, int and boolean data types:
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]
A list can contain different data types:
Example
A list with strings, integers and boolean values:
type[]
From Python's perspective, lists are defined as objects with the data type 'list':
Example
What is the data type of a list?
print[type[mylist]]
The list[] Constructor
It is also possible to use the list[] constructor when creating a new list.
Example
Using the list[] constructor to make a List:
print[thislist]
Python Collections [Arrays]
There are four collection data types in the Python programming language:
- List is a collection which is ordered and changeable. Allows duplicate members.
- Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
- Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
- Dictionary is a collection which is ordered** and changeable. No duplicate members.
*Set items are unchangeable, but you can remove and/or add items whenever you like.
**As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.