Python

Python Dictionaries

Python Dictionaries are one of the most powerful and flexible built-in data types in Python. They provide a way to store data in key-value pairs, making it easy to access, update, and manipulate structured information efficiently. Dictionaries are widely used in real-world applications, from configuration settings to storing user data and mapping relationships.

What are Python Dictionaries?

A Python Dictionary is an unordered, mutable collection of items. Each item is a pair of a key and its corresponding value. Keys must be unique and immutable (such as strings, numbers, or tuples), while values can be of any data type.

Basic Syntax of Python Dictionaries


# Creating a dictionary
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}
print(person["name"])  # Output: Alice

In this example, the dictionary person contains three key-value pairs, and you can retrieve values by referencing their keys.

Key Features of Python Dictionaries

Unordered: Items are not stored in any particular order (until Python 3.7; from 3.7 onwards, dictionaries maintain insertion order).
Mutable: You can add, change, or remove items.
Dynamic: Size and contents can be modified at runtime.
Efficient Lookup: Accessing a value by key is very fast, typically O(1) time complexity.

Creating Python Dictionaries

Different Ways to Create Dictionaries


# Using curly braces
d1 = {"a": 1, "b": 2}

# Using dict() constructor
d2 = dict(x=10, y=20)

# Using a list of tuples
d3 = dict([("name", "Bob"), ("age", 25)])

# Empty dictionary
empty = {}

Accessing and Modifying Dictionary Elements


person = {"name": "Tom", "age": 22}

# Accessing value
print(person["age"])  # Output: 22

# Modifying value
person["age"] = 23

# Adding a new key-value pair
person["city"] = "London"

Common Dictionary Methods

Method	Description	Example
keys()	Returns all keys	person.keys()
values()	Returns all values	person.values()
items()	Returns key-value pairs	person.items()
get()	Returns value for key, or default	person.get("name")
update()	Updates dictionary with key-value pairs	person.update({"age": 35})
pop()	Removes item with given key	person.pop("city")

Looping Through Python Dictionaries


user = {"name": "Eva", "role": "admin", "status": "active"}

# Loop through keys
for key in user:
    print(key)

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

# Loop through key-value pairs
for key, value in user.items():
    print(key, ":", value)

Nested Python Dictionaries

You can nest dictionaries within dictionaries for complex data structures.


students = {
    "student1": {"name": "Alice", "age": 21},
    "student2": {"name": "Bob", "age": 22}
}

print(students["student1"]["name"])  # Output: Alice

Why Use Python Dictionaries?

To store data in key-value pairs for quick access.
To represent structured information (e.g., user profiles, configurations).
To model real-world relationships like ID to name, product code to description.
To build JSON-like structures for APIs and databases.

Real-World Applications of Python Dictionaries

Storing user data in web applications
Representing JSON data
Building frequency counters and word count applications
Maintaining application settings or environment variables
Mapping keys to values in lookup tables