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Python Dictionaries

A dictionary in Python is an unordered, mutable collection that stores key-value pairs.

  • Each key must:
    • be unique
    • be associated with a specific value
  • defined using curly braces {} and specifying key-value pairs separated by colons :
  • Purpose:
    • Key-Value Mapping
      • allow you to associate values with descriptive keys, providing a convenient way to represent relationships between data
    • Fast Retrieval
      • Due to hashing, dictionaries offer fast and efficient retrieval of values based on keys
    • Flexibility
      • versatile for representing complex data structures, configurations, and mappings
  • Uses
    • ideal when you need to organize data in a way that facilitates efficient and expressive access
    • useful in network automation for managing information about devices, configurations, or any data that involves a key-value relationship
# Dictionary representing network devices
network_devices = {
    'Router1': '192.168.1.1',
    'Switch1': '10.0.0.1',
    'Firewall1': '172.16.0.1',
    'Server1': '192.168.2.1',
}
 
# Accessing information about a specific device
router_ip = network_devices['Router1']
 
# Adding a new device to the dictionary
network_devices['Switch2'] = '10.0.0.2'
 
# Resulting updated dictionary
# {'Router1': '192.168.1.1', 'Switch1': '10.0.0.1', 'Firewall1': '172.16.0.1', 'Server1': '192.168.2.1', 'Switch2': '10.0.0.2'}

Dictionary Data Types

  • key-value pairs can be of various data types
  • Common data types
    • Keys:
      • Keys can be of any immutable data type
        • e.g., integers, strings, or tuples
      • Lists and dictionaries cannot be used as keys
        • are mutable
    • Values:
      • Values can be of any data type

Dictionary Constructor

The dictionary constructor dict() is a built-in function that allows you to create a dictionary from various data structures.

  • can take no arguments, a dictionary, or an iterable containing key-value pairs
  • Role and function:
    1. creating empty dictionaries
    2. converting from iterables
    3. copying dictionaries
# Example: Creating a dictionary using the constructor for network devices
device_list = [('Router1', '192.168.1.1'), ('Switch1', '10.0.0.1'), ('Firewall1', '172.16.0.1')]
 
# Using the dictionary constructor to convert a list of tuples into a dictionary
network_devices = dict(device_list)
 
# Resulting dictionary: {'Router1': '192.168.1.1', 'Switch1': '10.0.0.1', 'Firewall1': '172.16.0.1'}

Access Dictionary Values

  • items in a dictionary can be accessed using their keys in quotes "" in brackets []
# Define a dictionary
dict1 = {"key1": "value1", "key2": "value2", "key3": "value3"}
 
# Access an item using its key
value1 = dict1["key1"]
  • If key does not exist
    • returns KeyError
  • Can use get method instead which returns None
    • e.g., value = dict1.get("non_existent_key")
    • can specify a default value as second parameter
      • e.g., value = dict1.get("non_existent_key", "default_value")

Modifying Dictionary Items

Change Dictionary Item

  • change the values in a dictionary by directly assigning a new value to a specific key
# Creating a dictionary
network_devices = {
    'Router1': '192.168.1.1',
    'Switch1': '10.0.0.1',
    'Firewall1': '172.16.0.1',
    'Server1': '192.168.2.1',
}
 
# Changing the value of a specific key
network_devices['Router1'] = '192.168.1.100'
  • nested dicts: nested_dict['outer_key']['inner_key'] = 'new_value'

Remove Item From Dictionary

  • Can remove an item from dictionary using:
    • del keyword
      • e.g., del dictionary['key1']
    • pop() method
      • returns the value of removed item
      • e.g., value = dictionary.pop('key1')

Update a Dictionary

The update() method is used to update a dictionary with the key/value pairs from another dictionary, or from an iterable of key/value pairs.

  • if a key in the second dictionary already exists in the first dictionary, the value for that key in the first dictionary will be updated
  • If a key in the second dictionary does not exist in the first dictionary, a new item with that key and value will be added to the first dictionary
# Define two dictionaries
dict1 = {"key1": "value1", "key2": "value2"}
dict2 = {"key2": "new_value2", "key3": "value3"}
 
# Update dict1 with dict2
dict1.update(dict2)
 
# New values:
# dict1 = {"key1": "value1", "key2": "new_value2", "key3": "value3"}

Adding Dictionary Items

To add an item to a Python dictionary, simply assign a value to a new key in the dictionary.

dictionary = {'key1': 'value1', 'key2': 'value2'}
dictionary['key3'] = 'value3'

Clearing a Dictionary

To clear a Python dictionary, the clear() method is used.

dictionary = {'key1': 'value1', 'key2': 'value2'}
dictionary.clear()

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