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.
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:
Python1# Dicitionary with grades as keys and lists of students as values 2school_directory = { 3 'Grade1': ['Amy', 'Bobby', 'Charlie'], 4 'Grade2': ['David', 'Eve', 'Frank'], 5 'Grade3': ['George', 'Hannah', 'Ivy'] 6} 7 8# Prints the Grade1 list in the dictionary 9print(school_directory['Grade1']) # Output: ['Amy', 'Bobby', 'Charlie']
Just like their non-nested versions, creating nested structures is straightforward.
Nested Dictionary:
Python1# Dictionary within a dictionary 2nested_dict = { 3 'fruit': { 4 'apple': 'red', # key-value pair within the 'fruit' dictionary 5 'banana': 'yellow' # another key-value pair within the 'fruit' dictionary 6 }, 7 'vegetable': { 8 'carrot': 'orange', 9 'spinach': 'green' 10 } 11} 12 13# Prints the nested dictionary 14print(nested_dict)
Nested List:
Python1# Lists within a list 2nested_list = [ 3 [1, 2, 3], # inner list within the outer list 4 [4, 5, 6], # another inner list within the outer list 5 [7, 8, 9] # third inner list within the outer list 6] 7 8# Prints the nested list 9print(nested_list)
Nested Dictionary and Lists:
Python1# Lists within a dictionary 2list_dict = { 3 'numbers': [1, 2, 3], # keys associated with lists 4 'letters': ['a', 'b', 'c'] 5} 6 7# Prints the dictionary of lists 8print(list_dict)
The retrieval of values from nested dictionaries
or lists
follows rules similar to those for their non-nested counterparts.
From Nested Dictionary:
Python1# Dictionary within a dictionary 2nested_dict = { 3 'fruit': { 4 'apple': 'red', # key-value pair within the 'fruit' dictionary 5 'banana': 'yellow' # another key-value pair within the 'fruit' dictionary 6 }, 7 'vegetable': { 8 'carrot': 'orange', 9 'spinach': 'green' 10 } 11} 12 13# Accessing apple's color from nested dictionary 14print(nested_dict['fruit']['apple']) # Output: 'red'
From Nested List:
Python1# Lists within a list 2nested_list = [ 3 [1, 2, 3], # inner list within the outer list 4 [4, 5, 6], # another inner list within the outer list 5 [7, 8, 9], # third inner list within the outer list 6] 7 8# Accessing the 3rd value from the 2nd list in nested list 9print(nested_list[1][2]) # Output: 6
From Both:
Python1# Lists within a dictionary 2list_dict = { 3 'numbers': [1, 2, 3], # keys associated with lists 4 'letters': ['a', 'b', 'c'] 5} 6 7# Accessing the second letter in the 'letters' list in list_dict 8print(list_dict['letters'][1]) # Output: 'b'
The modification of nested lists
and dictionaries
is similar to that of non-nested versions.
Python1# Modifying spinach's color to red 2nested_dict['vegetable']['spinach'] = 'red' 3 4# Adding 10 to the first list in nested list 5nested_list[0].append(10) 6 7# Adding cherry to the 'fruit' dictionary in nested_dict 8nested_dict['fruit']['cherry'] = 'red' 9 10# Deleting the 2nd value from the 3rd list in nested list 11del nested_list[2][1] 12 13# Deleting apple from the 'fruit' dictionary in nested_dict 14del nested_dict['fruit']['apple']
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.
Python1# Trying to print a non-existent key in nested_dict 2try: 3 print(nested_dict['fruit']['mango']) 4except KeyError: 5 print("Key not found!")
In this code:
try
block attempts to execute print(nested_dict['fruit']['mango'])
.mango
does not exist in nested_dict['fruit']
, a KeyError
is raised.except KeyError
block catches this error and prints "Key not found!" instead of stopping the program execution.Bravo! You've made a journey through nested lists
and dictionaries
, terms that are becoming increasingly common in the data-intensive programming world. We've learned how to create, access, and modify values in these complex structures and how to handle errors.
Up next, we have hands-on practice sessions to solidify your understanding of these concepts. Hold on to your hats!