Data Structures II: Dictionaries and Sets

This chapter concludes the introduction to Python’s core data structures by covering two essential unordered collections: Dictionaries (key-value pairs) and Sets (unique items).

1. Dictionaries (dict)

A Dictionary is an unordered collection of data values, used to store data in key: value pairs. Dictionaries are mutable and highly optimized for retrieving values based on a unique key.

A. Creating and Accessing Dictionaries

• Dictionaries are defined using curly braces {}.
• Keys must be unique and immutable (usually strings, numbers, or tuples). Values can be any data type.
• Values are accessed by referencing their associated key, not an index.

user_profile = {
    "name": "Alex",
    "age": 28,
    "city": "New York",
    "is_active": True
}

# Accessing a value using its key
print(user_profile["name"]) # Output: Alex

B. Modifying Dictionaries

Since dictionaries are mutable, you can add new pairs, change existing values, and remove pairs.

user_profile["email"] = "alex@example.com" # Adds a new key-value pair
user_profile["age"] = 29                   # Updates the existing age value

C. Dictionary Methods for Views

Python provides methods to retrieve views of the keys, values, and pairs for iteration.

# Iterating over all key-value pairs
for key, value in user_profile.items():
    print(f"{key}: {value}")

D. Safe Access with get()

Using bracket notation (dict[key]) raises a KeyError if the key doesn’t exist, which can crash your program. The get() method is a safer way to access values, returning None (or a specified default) if the key is missing.

# Safe access
country = user_profile.get("country", "Unknown") 
# Since 'country' doesn't exist, it returns 'Unknown'

2. Sets (set)

A Set is an unordered, mutable collection of unique elements. Sets are primarily used to efficiently test for membership or eliminate duplicate entries from a list.

A. Creating Sets

• Sets are defined using curly braces {} or the set() constructor.
• Note: To create an empty set, you must use empty_set = set(), as {} creates an empty dictionary.

my_set = {"apple", "banana", "cherry", "apple"} 

print(my_set) 
# Output: {'apple', 'banana', 'cherry'} (The duplicate 'apple' is removed)

B. Set Operations

Sets are highly efficient for performing mathematical set operations:

set_a = {1, 2, 3}
set_b = {3, 4, 5}

# Find common elements
common = set_a.intersection(set_b) 
print(common) # Output: {3}