Module 12 Practice Problems – Introduction to Python

Problem 1 Easy

What type of error would this code produce? print("Hello" + 5)

Think about what happens when you try to combine different data types.

TypeError - you cannot concatenate a string and an integer. Fix: print("Hello" + str(5)) or print(f"Hello{5}")

Problem 2 Easy

Write a try/except block that catches a ValueError when converting user input to an integer.

Put int(input(...)) in the try block and catch ValueError.

try:
    num = int(input("Enter a number: "))
    print(f"You entered {num}")
except ValueError:
    print("That's not a valid number!")

Problem 3 Easy

What type of error would my_list = [1, 2, 3]; print(my_list[5]) produce?

The list only has indices 0, 1, and 2.

IndexError: list index out of range - the list has 3 items (indices 0-2) but you tried to access index 5.

Problem 4 Easy

Write a try/except block that handles a ZeroDivisionError when dividing two numbers.

Put the division in the try block and catch ZeroDivisionError.

a = 10
b = 0
try:
    result = a / b
    print(f"Result: {result}")
except ZeroDivisionError:
    print("Cannot divide by zero!")

Problem 5 Easy

What type of error would pritn("hello") produce?

Look carefully at the function name.

NameError: name 'pritn' is not defined - it is a typo of print.

Problem 6 Medium

Write a try/except/else block. Try to open a file for reading. If FileNotFoundError occurs, print an error. If successful, print the content.

The else block runs only when no error occurs in the try block.

try:
    file = open("data.txt", "r")
except FileNotFoundError:
    print("File not found!")
else:
    content = file.read()
    print(content)
    file.close()

Problem 7 Medium

Write a function safe_divide(a, b) that returns the result of a/b, but returns None if b is zero (using try/except).

Catch ZeroDivisionError and return None in the except block.

def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        print("Cannot divide by zero!")
        return None

print(safe_divide(10, 3))   # 3.333...
print(safe_divide(10, 0))   # None

Problem 8 Medium

Write a function that validates an email address. It should raise a ValueError if the email does not contain "@".

Use if "@" not in email: and then raise ValueError(...).

def validate_email(email):
    if "@" not in email:
        raise ValueError(f"Invalid email: {email} (missing @)")
    return True

try:
    validate_email("user@example.com")  # OK
    validate_email("invalid-email")     # Raises error
except ValueError as e:
    print(e)

Problem 9 Medium

Write code that catches multiple exception types. Ask for two numbers and divide them. Handle both ValueError (non-numeric input) and ZeroDivisionError.

Use multiple except blocks, one for each error type.

try:
    a = float(input("Enter first number: "))
    b = float(input("Enter second number: "))
    result = a / b
    print(f"Result: {result}")
except ValueError:
    print("Please enter valid numbers!")
except ZeroDivisionError:
    print("Cannot divide by zero!")

Problem 10 Medium

Add print debugging to this buggy function. The function should return the sum of even numbers in a list, but it returns the wrong value.

def sum_evens(numbers):
    total = 0
    for n in numbers:
        if n % 2 == 0:
            total = n    # Bug here
    return total

Add print statements to see what total is after each iteration.

def sum_evens(numbers):
    total = 0
    for n in numbers:
        if n % 2 == 0:
            print(f"DEBUG: n={n}, total before={total}")
            total += n   # Fix: += instead of =
            print(f"DEBUG: total after={total}")
    return total

# The bug was using = instead of +=
print(sum_evens([1, 2, 3, 4, 6]))  # Should be 12

Problem 11 Medium

Write a try/except/finally block that opens a file, reads it, and ensures the file is always closed in the finally block.

Declare file = None before the try block so it exists in the finally block even if open fails.

file = None
try:
    file = open("data.txt", "r")
    content = file.read()
    print(content)
except FileNotFoundError:
    print("File not found!")
finally:
    if file is not None:
        file.close()
        print("File closed.")

Problem 12 Hard

Write a function get_integer(prompt) that keeps asking the user for input until they enter a valid integer. Use a while loop with try/except.

Use while True and return to exit the loop when valid input is received.

def get_integer(prompt):
    while True:
        try:
            return int(input(prompt))
        except ValueError:
            print("Please enter a valid integer.")

age = get_integer("Enter your age: ")
print(f"Your age is {age}")

Problem 13 Hard

Write a function that safely accesses a dictionary key. If the key does not exist, catch the KeyError and return a default value.

Use try/except KeyError, or compare with the built-in .get() method.

def safe_get(dictionary, key, default=None):
    try:
        return dictionary[key]
    except KeyError:
        return default

person = {"name": "Alice", "age": 30}
print(safe_get(person, "name"))           # Alice
print(safe_get(person, "email", "N/A"))  # N/A

Problem 14 Hard

Write a function process_scores(filename) that reads scores from a file, handles FileNotFoundError and ValueError, and returns the average score.

Use nested try/except - one for file operations, another for converting each line to a number.

def process_scores(filename):
    try:
        with open(filename, "r") as file:
            scores = []
            for line in file:
                try:
                    score = float(line.strip())
                    scores.append(score)
                except ValueError:
                    print(f"Skipping invalid: {line.strip()}")
            if scores:
                return sum(scores) / len(scores)
            return 0
    except FileNotFoundError:
        print(f"File '{filename}' not found!")
        return None

Problem 15 Hard

Find and fix all the bugs in this code. There are at least 4 errors.

def calculate_stats(numbers)
    if len(numbers) = 0:
        return None

    total = 0
    for num in numbers:
        total += num
    average = total / len(numbers)

    return "Average: " + average

print(calculate_stats([10, 20, 30]))

Look for: missing colon, wrong comparison operator, type mismatch in concatenation, and a possible indentation issue.

Bugs found:

Missing : after function definition
= should be == in the if condition
Cannot concatenate string and float - need str(average) or f-string

def calculate_stats(numbers):       # Fix 1: added :
    if len(numbers) == 0:            # Fix 2: = → ==
        return None

    total = 0
    for num in numbers:
        total += num
    average = total / len(numbers)

    return f"Average: {average}"     # Fix 3: use f-string

print(calculate_stats([10, 20, 30]))  # Average: 20.0

Module 12: Practice Problems