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'}

# Merge with | operator (Python 3.9+)
defaults = {"theme": "dark", "language": "en"}
user_prefs = {"theme": "light"}
settings = defaults | user_prefs
print(settings)  # {'theme': 'light', 'language': 'en'}

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Removing Items

student = {"name": "Alice", "age": 20, "grade": "A", "email": "alice@example.com"}

# del — delete a specific key
del student["email"]

# pop() — remove and return a value
grade = student.pop("grade")
print(grade)    # A
print(student)  # {'name': 'Alice', 'age': 20}

# pop() with default — no error if key missing
result = student.pop("phone", "not found")
print(result)  # not found

# popitem() — remove and return the last inserted item
student["city"] = "London"
last = student.popitem()
print(last)  # ('city', 'London')

# clear() — remove all items
student.clear()
print(student)  # {}

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Iterating Over Dictionaries

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

# Loop through keys (default)
for key in student:
    print(key)

# Loop through values
for value in student.values():
    print(value)

# Loop through key-value pairs
for key, value in student.items():
    print(f"{key}: {value}")
# name: Alice
# age: 20
# grade: A

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Dictionary Methods

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

# keys(), values(), items()
print(list(student.keys()))    # ['name', 'age', 'grade']
print(list(student.values()))  # ['Alice', 20, 'A']
print(list(student.items()))   # [('name', 'Alice'), ('age', 20), ('grade', 'A')]

# Check if a key exists
print("name" in student)       # True
print("email" in student)      # False

# setdefault() — get value or set it if missing
student.setdefault("email", "unknown")
print(student["email"])  # unknown

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Dictionary Comprehensions

# Create a dictionary from a loop
squares = {x: x**2 for x in range(6)}
print(squares)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# Filter a dictionary
scores = {"Alice": 85, "Bob": 62, "Charlie": 91, "Diana": 78}
high_scores = {name: score for name, score in scores.items() if score >= 80}
print(high_scores)  # {'Alice': 85, 'Charlie': 91}