Dictionary Variables in Python

In Python, dictionaries are a special type of data structure that store a collection of objects, each accessible via a unique key. Each key is associated with a value.

variable = { key1: value1, ... , keyN: valueN }

Technically, this is known as an associative array.

What is an associative array?

Each element in the dictionary is a pair consisting of a key and a value, separated by a colon.

The pairs in the list are separated by commas.

Note: A dictionary key can be an integer, a string, or a tuple.

A Practical Example

To create an 'employees' variable, use the dict() function.

employees = dict()

Then, assign elements and keys to the variable.

employees = { 'Andrea':1182, 'Paolo':1034, 'Mario':1248 }

The dictionary contains the names of some employees and their respective ID numbers.

In the next line, we retrieve the element with the key 1182.

So, if you type

employees['Andrea']

The interpreter fetches the value assigned to the key 'Andrea' in the "employees" dictionary and displays it.

The output will be:

1182

This demonstrates how to use dictionary variables in Python.

An Alternative Way to Create Dictionaries in Python

Python allows you to create a dictionary by spreading the assignment operations across multiple lines.

Dictionary={}
Dictionary['Andrea']=1182
Dictionary['Paolo']=1034
Dictionary['Mario']=1248

The end result is the same.

In this approach, the key-value pairs are assigned on different lines.

This method is particularly useful in loops or when the dictionary contains numerous pairs.

How to Display Dictionary Keys

To list all the keys in the dictionary, use the keys method:

employees.keys()

The result of this command is:

dict_keys(['Andrea', 'Paolo', 'Mario'])

These are the keys in the employees dictionary.

How to Add a Key to the Dictionary

To add a new key to the dictionary variable, perform an assignment operation.

On the left of the equals sign, specify the variable and the key in square brackets. On the right, specify the value to assign.

variable['key'] = value

A Practical Example

To assign the value 1182 to the key 'Andrea' in the Dictionary variable, write

Dictionary['Andrea'] = 1182

How to Check for a Key's Presence

To check if a key is present in the dictionary, use the in operator.

'Andrea' in employees

The result is:

true

The key 'Andrea' is indeed present in the employees dictionary.

How to Check for a Key's Absence?

Just add the not operator.

'Andrea' not in employees

How to Retrieve a Key's Value

To retrieve the value of a dictionary key, type the variable name with the key in square brackets.

dictionary['key']

The interpreter recognizes this as a dictionary and displays the value associated with the key.

A Practical Example

To retrieve the value assigned to the key 'Andrea' in the 'employees' dictionary, write

employees['Andrea']

The output will be:

1182

How to Remove an Element from the Dictionary

To delete an element from the dictionary, use

del employees['Andrea']

This command removes the key 'Andrea' from the employees dictionary.

How to Count the Elements in a Dictionary

To get the number of elements in a dictionary, use the len function.

len(dictionary_name)

A Practical Example

To get the number of elements in the employees dictionary, type:

len(employees)

The output will be:

3

How to Create an Empty Dictionary

To create an empty dictionary, assign two curly braces { } to the variable.

dictionary = {}

The interpreter creates a dictionary with no elements inside.

Note: Empty curly braces are an alternative way to declare a dictionary variable. When the interpreter recognizes them, it automatically declares the variable as a dictionary.

Dictionary Methods

Dictionaries in Python come with several useful methods.

The main methods for dictionary variables are:

• s.clear()
  Removes all elements from the dictionary s.
• s.copy()
  Creates a new dictionary that is a copy of s.
• s.get(key, default)
  Retrieves the value associated with key from the dictionary. If the key is not present, it returns the default value.
• s.keys()
  Displays all the keys of the dictionary s.
• s.items()
  Displays all the key-value pairs of the dictionary s.
• s.setdefault(key, default)
  Retrieves the value associated with key. If the key is not present, it returns the default value and adds the key-value pair to the dictionary.
• s.values()
  Displays all the values in the dictionary s.

Dictionary Comprehension

Comprehensions can also be applied to dictionaries.

However, unlike sets, dictionaries require working with key-value pairs.

A Practical Example

provinces = { 'Rome':'RM', 'Milan':'MI' }
abbreviations = { value:key for key, value in provinces.items() }

The above comprehension creates a new 'abbreviations' dictionary by swapping the keys and values of the 'provinces' dictionary.