Dictionaries in Python

A dictionary in Python is an unordered collection of key-value pairs. It is defined using curly braces {} and provides an efficient way to store and retrieve data using unique keys.

Creating a Dictionary

Dictionaries are created using curly braces {} or the dict() constructor.

# Using curly braces
student = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

# Using the dict() constructor
person = dict(name="John", age=30, city="San Francisco")

print(student) # {'name': 'Alice', 'age': 25, 'city': 'New York'}
print(person) # {'name': 'John', 'age': 30, 'city': 'San Francisco'}

Accessing Elements in a Dictionary

You can access dictionary values using keys.

student = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

# Using square brackets
print(student["name"])  # Alice

# Using the get() method (prevents KeyError)
print(student.get("age"))  # 25
print(student.get("gender", "Not found"))  # Not found

Updating a Dictionary

You can modify dictionary values by accessing them through keys.

student = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

student["age"] = 26
student["city"] = "Boston"
print(student)  # {'name': 'Alice', 'age': 26, 'city': 'Boston'}

- Adding New Elements

You can add new key-value pairs.

student = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

student["course"] = "Computer Science"
print(student)
# {'name': 'Alice', 'age': 25, 'city': 'New York', 'course': 'Computer Science'}

- Removing Elements from a Dictionary

You can remove dictionary elements using del, pop(), popitem(), and clear().

student = {
    "name": "Alice",
    "age": 25,
    "city": "New York",
    "gender": "Female",
}

# Using del
del student["age"]

# Using pop()
course = student.pop("gender")  # Removes and returns value

# Using popitem() (removes last inserted item)
last_item = student.popitem()

# Using clear() (removes all items)
student.clear()

print(student)  # {}

Iterating Over a Dictionary

You can iterate over a dictionary using loops.

student = {
    "name": "Alice",
    "age": 25,
    "city": "New York",
    "gender": "Female",
}

# Iterating Over Keys
for key in student:
    print(key, student[key])

# Iterating Over Values
for value in student.values():
    print(value)

# Iterating Over Key-Value Pairs
for key, value in student.items():
    print(key, value)

Checking Key Existence

student = {
    "name": "Alice",
    "age": 25,
    "city": "New York",
    "gender": "Female",
}

if "name" in student:
    print(f"Name: {student["name"]}") # Name: Alice

Dictionary Comprehension

You can create dictionaries using dictionary comprehension.

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

Nesting in Dictionaries

Dictionaries can store lists or other dictionaries.

# Nesting Lists in Dictionaries
students = {
    "Alice": ["Math", "Science"],
    "Bob": ["History", "Geography"]
}

print(students["Alice"]) # ['Math', 'Science']

# Nesting Dictionaries in Dictionaries
class_data = {
    "Alice": {"age": 25, "city": "New York"},
    "Bob": {"age": 22, "city": "Chicago"}
}

print(class_data["Alice"]["city"])  # New York