Python Dictionaries

Dictionary

Dictionaries are used to store data values in key:value pairs.

A dictionary is a collection which is ordered*, changeable and do not allow duplicates.

Dictionaries are written with curly brackets, and have keys and values:

Dictionary Items

Dictionary items are ordered, changeable, and do not allow duplicates.

Dictionary items are presented in key:value pairs, and can be referred to by using the key name.

Ordered or Unordered?

When we say that dictionaries are ordered, it means that the items have a defined order, and that order will not change.

Unordered means that the items do not have a defined order, you cannot refer to an item by using an index.

Changeable

Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.

Duplicates Not Allowed

Dictionaries cannot have two items with the same key:

Dictionary Length

To determine how many items a dictionary has, use the len() function:

Dictionary Items - Data Types

The values in dictionary items can be of any data type:

type()

From Python's perspective, dictionaries are defined as objects with the data type 'dict':

The dict() Constructor

It is also possible to use the dict() constructor to make a dictionary.

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.

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.