A dictionary in Python is a collection of key-value pairs. It is unordered, mutable, and indexed, making it an incredibly versatile data structure for storing and managing related data.

Key Features of Dictionaries

1. Key-Value Pairs: Each element in a dictionary is a pair consisting of a unique key and its associated value.
2. Keys are Unique: Duplicate keys are not allowed. If you assign a value to an existing key, the old value is overwritten.
3. Mutable: You can modify, add, or remove key-value pairs.
4. Unordered (Pre-Python 3.7): In Python 3.7+, dictionaries maintain the insertion order.
5. Keys are Immutable: Keys must be of an immutable type (e.g., strings, numbers, or tuples).

Creating a Dictionary

You can create a dictionary using curly braces {} or the dict() constructor.

# Using curly braces
student = {"name": "John", "age": 25, "grade": "A"}

# Using the dict() constructor
person = dict(name="Alice", age=30, profession="Engineer")

# Empty dictionary
empty_dict = {}

Accessing Dictionary Values

You can access values in a dictionary using their corresponding keys.

student = {"name": "John", "age": 25, "grade": "A"}

# Access value using a key
print(student["name"])  # Output: John

# Using the get() method (avoids KeyError if key doesn't exist)
print(student.get("age"))  # Output: 25
print(student.get("school", "Not Found"))  # Output: Not Found

Modifying a Dictionary

Adding/Updating Elements

student = {"name": "John", "age": 25}

# Add a new key-value pair
student["grade"] = "A"

# Update an existing key's value
student["age"] = 26

print(student)  # Output: {'name': 'John', 'age': 26, 'grade': 'A'}

Removing Elements

student = {"name": "John", "age": 25, "grade": "A"}

# Remove an item using pop()
age = student.pop("age")
print(age)  # Output: 25
print(student)  # Output: {'name': 'John', 'grade': 'A'}

# Remove an item using del
del student["grade"]
print(student)  # Output: {'name': 'John'}

# Remove all items using clear()
student.clear()
print(student)  # Output: {}

Dictionary Methods

Iterating Through a Dictionary

You can iterate through a dictionary’s keys, values, or items (key-value pairs).

student = {"name": "John", "age": 25, "grade": "A"}

# Iterate through keys
for key in student:
    print(key)  # Output: name, age, grade

# Iterate through values
for value in student.values():
    print(value)  # Output: John, 25, A

# Iterate through key-value pairs
for key, value in student.items():
    print(f"{key}: {value}")
    # Output:
    # name: John
    # age: 25
    # grade: A

Nested Dictionaries

Dictionaries can contain other dictionaries, which is useful for representing structured data.

students = {
    "student1": {"name": "John", "age": 25},
    "student2": {"name": "Alice", "age": 22},
}

# Access nested dictionary
print(students["student1"]["name"])  # Output: John

# Add a new nested dictionary
students["student3"] = {"name": "Bob", "age": 23}
print(students)

Dictionary Comprehension

Python provides a concise way to create dictionaries using dictionary comprehension.

# Create a dictionary with squares of numbers
squares = {x: x**2 for x in range(1, 6)}
print(squares)  # Output: {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Common Use Cases of Dictionaries

1. Storing Configurations: config = {"theme": "dark", "font": "Arial", "font_size": 12}
2. Counting Occurrences: text = "hello world" char_count = {} for char in text: char_count[char] = char_count.get(char, 0) + 1 print(char_count) # Output: {'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, 'r': 1, 'd': 1}
3. Database Representation: employee = {"id": 101, "name": "John", "position": "Manager"}

Comparison with Lists

Conclusion

Dictionaries are a fundamental part of Python, offering powerful tools to manage data with key-value pairs. They are especially useful for structured data, quick lookups, and complex data representations.