Navigating the Nested Wilderness: Mastering Python's Compound Data Structures | CodeSignal Learn

Navigating the Nested Wilderness: Mastering Python's Compound Data Structures | CodeSignal Learn

Introduction

Welcome to our exploration of Compound Data Structures in Python . Having navigated through Dictionaries, Sets, and Tuples, we'll delve into nested dictionaries and lists . These structures enable us to handle complex and hierarchical data, which is typical in real-world scenarios. This lesson will guide you through a recap of the basics, the creation and modification of nested dictionaries and lists, as well as common error handling.

Recap: Dictionaries, Lists, and Understanding Nested Structures

As a quick recap, Lists are mutable, ordered collections, while Dictionaries are collections of key-value pairs. These structures can be nested. Here's a simple example of a school directory: Python# Dicitionary with grades as keys and lists of students as values school_directory = { 'Grade1': ['Amy', 'Bobby', 'Charlie'], 'Grade2': ['David', 'Eve', 'Frank'], 'Grade3': ['George', 'Hannah', 'Ivy'] } # Prints the Grade1 list in the dictionary print(school_directory['Grade1']) # Output: ['Amy', 'Bobby', 'Charlie']

Creating Nested Dictionaries and Lists

Just like their non-nested versions, creating nested structures is straightforward. Nested Dictionary: Python# Dictionary within a dictionary nested_dict = { 'fruit': { 'apple': 'red', # key-value pair within the 'fruit' dictionary 'banana': 'yellow' # another key-value pair within the 'fruit' dictionary }, 'vegetable': { 'carrot': 'orange', 'spinach': 'green' } } # Prints the nested dictionary print(nested_dict) Nested List: Python# Lists within a list nested_list = [ [1, 2, 3], # inner list within the outer list [4, 5, 6], # another inner list within the outer list [7, 8, 9] # third inner list within the outer list ] # Prints the nested list print(nested_list) Nested Dictionary and Lists: Python# Lists within a dictionary list_dict = { 'numbers': [1, 2, 3], # keys associated with lists 'letters': ['a', 'b', 'c'] } # Prints the dictionary of lists print(list_dict)

Accessing Values in Nested Structures

The retrieval of values from nested dictionaries or lists follows rules similar to those for their non-nested counterparts. From Nested Dictionary: Python# Dictionary within a dictionary nested_dict = { 'fruit': { 'apple': 'red', # key-value pair within the 'fruit' dictionary 'banana': 'yellow' # another key-value pair within the 'fruit' dictionary }, 'vegetable': { 'carrot': 'orange', 'spinach': 'green' } } # Accessing apple's color from nested dictionary print(nested_dict['fruit']['apple']) # Output: 'red' From Nested List: Python# Lists within a list nested_list = [ [1, 2, 3], # inner list within the outer list [4, 5, 6], # another inner list within the outer list [7, 8, 9], # third inner list within the outer list ] # Accessing the 3rd value from the 2nd list in nested list print(nested_list[1][2]) # Output: 6 From Both: Python# Lists within a dictionary list_dict = { 'numbers': [1, 2, 3], # keys associated with lists 'letters': ['a', 'b', 'c'] } # Accessing the second letter in the 'letters' list in list_dict print(list_dict['letters'][1]) # Output: 'b'

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Common Operations on these Structures

The modification of nested lists and dictionaries is similar to that of non-nested versions. # Modifying spinach's color to red nested_dict['vegetable']['spinach'] = 'red' # Adding 10 to the first list in nested list nested_list[0].append(10) # Adding cherry to the 'fruit' dictionary in nested_dict nested_dict['fruit']['cherry'] = 'red' # Deleting the 2nd value from the 3rd list in nested list del nested_list[2][1] # Deleting apple from the 'fruit' dictionary in nested_dict del nested_dict['fruit']['apple']

Identifying Errors and Handling Exceptions

Python executes exceptions effortlessly by providing the try / except blocks. The try block lets you test a block of code for errors, and the except block lets you handle the error. In the following example, we attempt to print a non-existent key in nested_dict . If the key is not found, a KeyError is raised, and the except block catches this error and executes its code. # Trying to print a non-existent key in nested_dict try: print(nested_dict['fruit']['mango']) except KeyError: print("Key not found!") In this code: The try block attempts to execute print(nested_dict['fruit']['mango']) . If mango does not exist in nested_dict['fruit'], a KeyError is raised. The except KeyError block catches this error and prints "Key not found!" instead of stopping the program execution.

Python

# Modifying spinach's color to red
nested_dict['vegetable']['spinach'] = 'red'

# Adding 10 to the first list in nested list
nested_list[0].append(10)  

# Adding cherry to the 'fruit' dictionary in nested_dict
nested_dict['fruit']['cherry'] = 'red'

# Deleting the 2nd value from the 3rd list in nested list
del nested_list[2][1]  

# Deleting apple from the 'fruit' dictionary in nested_dict
del nested_dict['fruit']['apple']