Python Dictionnaries

Python Dictionaries

Dictionaries are one of the most powerful and flexible data types in Python. They allow you to store and organize data using key-value pairs, making it easy to access and modify values based on a specific identifier.

What is a Dictionary?

A dictionary in Python is a collection of unordered, changeable, and indexed data. Unlike lists or tuples that use numerical indexes, dictionaries use keys to access their corresponding values.

Syntax:

my_dict = {
    "key1": "value1",
    "key2": "value2"
}

• Keys must be unique and immutable (like strings or numbers).
• Values can be of any data type and can be duplicated.

Creating a Dictionary

Here’s how you can define a dictionary:

person = {
    "name": "Alice",
    "age": 30,
    "city": "Paris"
}

You can also use the dict() constructor:

person = dict(name="Alice", age=30, city="Paris")

Accessing Values

To get the value of a specific key, use square brackets or the .get() method:

print(person["name"])   # Output: Alice
print(person.get("age"))  # Output: 30

• [] raises an error if the key doesn’t exist.
• .get() returns None (or a default value if provided) when the key is missing.

Adding and Updating Entries

You can add a new key-value pair or update an existing one:

person["email"] = "alice@example.com"  # Add new key
person["age"] = 31                    # Update existing key

Output:

{'name': 'Alice', 'age': 31, 'city': 'Paris', 'email': 'alice@example.com'}

Removing Entries

Python offers several ways to remove items from a dictionary:

• del keyword
• .pop() method
• .popitem() method

del person["city"]
print(person)

email = person.pop("email")
print(email)

last_item = person.popitem()  # Removes the last inserted pair
print(last_item)

Checking if a Key Exists

Use the in keyword to check if a key is present:

if "name" in person:
    print("Name is defined")

Iterating Over a Dictionary

You can loop through keys, values, or both:

# Keys
for key in person:
    print(key)

# Values
for value in person.values():
    print(value)

# Key-Value pairs
for key, value in person.items():
    print(f"{key} : {value}")

Dictionary Length

Use len() to get the number of key-value pairs:

print(len(person))  # Output: 2

Nested Dictionaries

A dictionary can contain another dictionary:

users = {
    "alice": {"age": 30, "email": "alice@example.com"},
    "bob": {"age": 25, "email": "bob@example.com"}
}

print(users["alice"]["email"])

Output:

alice@example.com

Summary

• A dictionary stores data using key-value pairs.
• Keys must be unique and immutable.
• Values can be of any type.
• You can easily add, update, delete, and iterate over items.
• They are perfect for structured data and quick lookups.

Dictionaries are an essential part of Python and mastering them will unlock more advanced programming techniques.