Python Dictionaries: Organizing Key-Value Data

Dictionaries in Python are powerful data structures that allow you to store and retrieve data using key-value pairs. They’re like real-world dictionaries, mapping a word (the key) to its definition (the value). This tutorial will guide you through creating, accessing, and manipulating dictionaries in Python.

What are Dictionaries?

A dictionary is a mutable (changeable) data structure that holds data in key-value pairs. Keys must be unique and immutable (like strings, numbers, or tuples), while values can be of any data type.

Example 1: Creating and Accessing a Simple Dictionary

Let’s create a dictionary to store the names and ages of a few friends.

# Create a dictionary
friends = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 35
}

# Accessing values using keys
alice_age = friends["Alice"]
print(alice_age)

# Accessing values using keys
bob_age = friends["Bob"]
print(bob_age)

# Accessing values using keys
charlie_age = friends["Charlie"]
print(charlie_age)

# Demonstrating error handling - trying to access a non-existent key
# This will raise a KeyError
# try:
#     david_age = friends["David"]
# except KeyError as e:
#     print(f"Error: Key '{e}' not found in the dictionary.")


In this example, we first create a dictionary named `friends` with three key-value pairs.  The keys are the names of our friends (strings), and the values are their ages (integers). We then access the values using their corresponding keys.  A `KeyError` is raised when a key that isn't present in the dictionary is attempted to be accessed. Using a `try...except` block to handle the error gracefully is a common best practice.


# Create a dictionary
friends = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 35
}

# Accessing values using keys
alice_age = friends["Alice"]
print(alice_age)

# Accessing values using keys
bob_age = friends["Bob"]
print(bob_age)

# Accessing values using keys
charlie_age = friends["Charlie"]
print(charlie_age)


Output:


30
25
35

Example 2: Adding and Modifying Dictionary Elements
Now, let's add a new friend to the dictionary and modify an existing age.

# Create a dictionary
friends = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 35
}

# Adding a new element
friends["David"] = 28

# Modifying an existing element
friends["Alice"] = 31

# Printing the updated dictionary
print(friends)


Here, we add a new key-value pair ("David": 28) to the `friends` dictionary. We then modify the age of "Alice" to 31. Dictionaries are mutable, so these changes are reflected immediately.


# Create a dictionary
friends = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 35
}

# Adding a new element
friends["David"] = 28

# Modifying an existing element
friends["Alice"] = 31

# Printing the updated dictionary
print(friends)


Output:


{'Alice': 31, 'Bob': 25, 'Charlie': 35, 'David': 28}

Example 3: Iterating Through a Dictionary
Let's iterate through the dictionary to print the names and ages of all our friends.

# Create a dictionary
friends = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 35
}

# Iterating through the dictionary using a for loop
for name, age in friends.items():
    print(f"{name} is {age} years old.")


The `friends.items()` method returns a view object containing key-value pairs from the dictionary.  We use a `for` loop to unpack each pair into the `name` and `age` variables. This is a concise and Pythonic way to iterate through dictionaries.


# Create a dictionary
friends = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 35
}

# Iterating through the dictionary using a for loop
for name, age in friends.items():
    print(f"{name} is {age} years old.")


Output:


Alice is 30 years old.
Bob is 25 years old.
Charlie is 35 years old.

Key Concepts Recap
Here's a quick recap of the key concepts covered in this tutorial:

Dictionaries store data in key-value pairs.
Keys must be unique and immutable.
Values can be of any data type.
You can access values using their keys.
You can add or modify elements using the assignment operator.
You can iterate through dictionaries using `for` loops and the `.items()` method.