Tuples, Sets, and Dictionaries

# CHAPTER 8

Tuples, Sets, and Dictionaries #

1. Chapter Introduction #

2. Tuples: Immutable Sequences #

A Tuple is exactly like a list, except it CANNOT be changed (Immutable) after it is created. They use parentheses (). We use tuples for fixed data, like geographic coordinates or RGB colors.

# Creating a tuple
coordinates = (40.7128, -74.0060)

# Accessing works just like a list
print(coordinates[0]) # 40.7128

# ERROR! Tuples do not support reassignment
# coordinates[0] = 41.0

3. Sets: Unordered and Unique #

A Set is an unordered collection with NO duplicate elements. They use curly braces {}. Sets are incredibly fast for membership testing (in) and mathematically comparing datasets (Unions, Intersections).

# Creating a set (Notice the duplicates)
user_ids = {101, 102, 103, 101, 101}

# Duplicates are automatically removed!
print(user_ids) # Output: {101, 102, 103}

# Fast membership testing
print(102 in user_ids) # Output: True

# Set Operations
group_A = {"Alice", "Bob", "Charlie"}
group_B = {"Bob", "Dave"}

print(group_A.intersection(group_B)) # Who is in both? {'Bob'}
print(group_A.union(group_B))        # Combine them all (no dupes)

*Pro-Tip: If you have a messy List with duplicates, convert it to a Set, then back to a List to clean it instantly: clean_list = list(set(messy_list)).*

4. Dictionaries: Key-Value Pairs #

A Dictionary maps a "Key" to a "Value", much like a real dictionary maps a word to its definition. They use curly braces {} with a colon : separating the key and value. This is Python's most important structure for dealing with JSON data and Web APIs.

# Creating a dictionary
customer = {
    "id": 554,
    "name": "David",
    "email": "david@example.com"
}

# Accessing data using the Key
print(customer["name"]) # Output: David

# Adding or updating a Key-Value pair
customer["status"] = "Active"
customer["email"] = "dave_new@example.com"

# Getting a list of all keys or values
print(customer.keys())
print(customer.values())

5. Nested Data Structures #

In data science, structures are often nested. A JSON response from a web server is usually a List of Dictionaries.

database = [
    {"user": "Alice", "role": "Admin", "scores": [90, 95, 88]},
    {"user": "Bob", "role": "Editor", "scores": [70, 75, 80]}
]

# How to access Bob's first score:
# 1. Access the 2nd item in the list (index 1)
# 2. Access the "scores" key in that dictionary
# 3. Access the 1st item in the scores list (index 0)
bobs_first_score = database[1]["scores"][0]
print(f"Bob's first score is {bobs_first_score}") # 70

6. Dictionary Comprehensions #

Just like List Comprehensions, you can create dictionaries in a single line.

names = ["Alice", "Bob", "Charlie"]

# Create a dictionary mapping the name to the length of the name
name_lengths = {name: len(name) for name in names}
print(name_lengths) # {'Alice': 5, 'Bob': 3, 'Charlie': 7}

7. Mini Project: Student Records Manager #

Let's use a dictionary to manage student grades and calculate their average.

# A dictionary where Keys are student names, Values are lists of scores
grade_book = {
    "Alice": [90, 95, 92],
    "Bob": [70, 65, 80],
    "Charlie": [85, 88, 90]
}

# Iterate through the dictionary (.items() gets both key and value)
for student, scores in grade_book.items():
    avg = sum(scores) / len(scores)
    
    if avg >= 90:
        status = "Honors"
    else:
        status = "Pass"
        
    print(f"{student}: Avg = {avg:.1f} | Status: {status}")

8. Common Mistakes #

• Indexing a Set: Because Sets are unordered, they don't have indexes. my_set[0] will throw an error.

• KeyError in Dictionaries: If you try to access customer["phone"] but the "phone" key doesn't exist, Python crashes. Use the .get() method instead: customer.get("phone", "Not Found") returns "Not Found" instead of crashing.

9. MCQs #

10. Interview Questions #

• Q: Explain the difference between a List and a Set. Give a real-world scenario where you would use a Set.

• Q: You have a dictionary of 10,000 customers. What happens if you try to access a key that doesn't exist? How do you write defensive code to prevent a crash?

11. Summary #

12. Next Chapter Recommendation #