Python Dictionaries

Python Dictionaries: Master Key-Value Pairs for Efficient Web Development

Welcome to Whitewood Media's Python programming tutorial series! In this lesson, we will focus on Python dictionaries, a versatile and powerful data structure that allows you to store and manage data as key-value pairs. Dictionaries are invaluable for web developers working with structured data or needing to perform fast lookups. In this tutorial, we'll define Python dictionaries, demonstrate their usage through code examples, and provide example problems to help you practice your newfound skills.

Definition of Python Dictionaries:

A Python dictionary is an unordered collection of key-value pairs. Keys must be unique and hashable, such as numbers, strings, or tuples, while values can be any data type, including other dictionaries or lists. Dictionaries are created using curly braces {} with key-value pairs separated by colons or the built-in dict() constructor.

Here's an example of a dictionary containing the names and ages of different people:

ages = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 22
}

Working with Python Dictionaries:

1. Accessing Values:

You can access the value associated with a specific key using square brackets [].

ages = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 22
}

print(ages["Alice"])  # Output: 30

If you try to access a non-existent key, a KeyError will be raised. To avoid this, you can use the get() method, which returns a default value if the key is not found.

print(ages.get("David", "Not found"))  # Output: Not found

2. Adding and Modifying Values:

You can add a new key-value pair or modify an existing one by assigning a value to a key using the = operator.

ages = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 22
}

# Adding a new key-value pair
ages["David"] = 35

# Modifying an existing key-value pair
ages["Alice"] = 31

print(ages)  # Output: {'Alice': 31, 'Bob': 25, 'Charlie': 22, 'David': 35}

3. Removing Key-Value Pairs:

To remove a key-value pair from a dictionary, use the del statement.

ages = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 22,
    "David": 35
}

del ages["Charlie"]
print(ages)  # Output: {'Alice': 30, 'Bob': 25, 'David': 35}

4. Looping Through a Dictionary:

You can loop through a dictionary using a for loop, which iterates over the keys by default.

ages = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 22
}

for name in ages:
    print(f"{name}: {ages[name]}")

To loop through key-value pairs, use the items() method:

for name, age in ages.items():
    print(f"{name}: {age}")

Summed Up

Example Problems with Dictionaries in Python:

Now that you have a solid understanding of Python dictionaries, try solving these example problems to practice your skills:

1. Write a function that takes a dictionary of names and ages and returns a new dictionary containing only the names of people aged 30 or older.

def filter_older_than_30(ages_dict):
    return {name: age for name, age in ages_dict.items() if age >= 30}

ages = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 22,
    "David": 35
}

older_people = filter_older_than_30(ages)
print(older_people)  # Output: {'Alice': 30, 'David': 35}

2. Write a function that takes a list of words and returns a dictionary where the keys are the words and the values are the lengths of the words.

def word_lengths(words):
    return {word: len(word) for word in words}

words = ["apple", "banana", "cherry", "date"]
lengths = word_lengths(words)
print(lengths)  # Output: {'apple': 5, 'banana': 6, 'cherry': 6, 'date': 4}

3. Write a function that takes a dictionary and returns the number of key-value pairs in the dictionary.

def count_pairs(dictionary):
    return len(dictionary)

ages = {
    "Alice": 30,
    "Bob": 25,
    "Charlie": 22
}

count = count_pairs(ages)
print(count)  # Output: 3

FAQs and Answers about Dictionaries in Python:

Q: Can I store elements of different data types in a dictionary?

A: Yes, a Python dictionary can store keys of any hashable data type, while values can be any data type, including other dictionaries, lists, or tuples.

Q: What is the difference between a dictionary and a list in Python?

A: The primary difference is that dictionaries are unordered collections of key-value pairs, while lists are ordered collections of elements. Additionally, dictionaries use curly braces {}, while lists use square brackets [].

Q: Can I have a dictionary inside another dictionary?

A: Yes, a dictionary can have another dictionary as a value. This is known as a nested dictionary.

Q: How can I merge two dictionaries?

A: You can merge two dictionaries using the update() method or the ** unpacking operator available in Python 3.5+.