Dictionaries and Sets

Dictionaries and sets are two more essential Python data structures. A dictionary stores data as key-value pairs — like a real dictionary maps words to definitions. A set stores unique, unordered elements — useful for membership testing and eliminating duplicates.

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Creating Dictionaries

# Curly braces with key: value pairs
student = {
    "name": "Alice",
    "age": 20,
    "grade": "A",
    "courses": ["Maths", "Physics"]
}

# dict() constructor
config = dict(host="localhost", port=8080, debug=True)

# Empty dictionary
empty = {}

print(student)
print(config)

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Accessing Values

student = {"name": "Alice", "age": 20, "grade": "A"}

# Square bracket notation
print(student["name"])  # Alice

# .get() — returns None (or a default) instead of raising an error
print(student.get("age"))         # 20
print(student.get("email"))       # None
print(student.get("email", "N/A"))  # N/A — custom default

# Square brackets raise KeyError for missing keys
# print(student["email"])  # KeyError: 'email'

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Adding and Modifying Values

student = {"name": "Alice", "age": 20}

# Add a new key-value pair
student["email"] = "alice@example.com"

# Modify an existing value
student["age"] = 21

# update() — merge another dictionary
student.update({"grade": "A", "age": 22})
print(student)
# {'name': 'Alice', 'age': 22, 'email': 'alice@example.com', 'grade': 'A'}