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Exercise Set 2: Intermediate Practice (OOP, Files, Errors, Comprehensions)

Relax. We'll talk through this in plain words — no textbook voice.

What you'll walk away with

  • Work through Exercise Set 2: Intermediate Practice (OOP, Files, Errors, Comprehensions) on your own
  • Practice the skills you've already learned and lock them in
  • Get comfortable finding bugs, fixing them, and checking your own work

Quick Thought

This exercise set takes things up a notch from Exercise Set 1, giving you a chance to combine Intermediate/Advanced chapter concepts like OOP (classes), error handling, file handling, and comprehensions into real hands-on problem-solving. In real projects, you rarely get to work with just one concept at a time — classes, exceptions, and file I/O usually show up together. That's why these tasks are designed to feel closer to that kind of real-world scenario. Solving them yourself and running the code will also sharpen your debugging skills.

Exercises

Task 1: Write a BankAccount class - it should have a balance attribute and two methods, deposit() and withdraw(). If someone tries to withdraw more than the balance, raise a custom exception called InsufficientFundsError. Task 2: Use try-except-finally to read a text file - if the file doesn't exist, catch the FileNotFoundError and print a friendly message. Either way, the finally block should always print "Read attempt finished". Task 3: Use a dictionary comprehension to build a {word: length} dictionary from a list of words, then sort and print the resulting dictionary by length.

Sample Code

python
# Task 1: BankAccount with custom exception
class InsufficientFundsError(Exception):
    pass

class BankAccount:
    def __init__(self, balance=0):
        self.balance = balance

    def deposit(self, amount):
        self.balance += amount

    def withdraw(self, amount):
        if amount > self.balance:
            raise InsufficientFundsError("Not enough balance!")
        self.balance -= amount

acc = BankAccount(1000)
acc.deposit(500)
try:
    acc.withdraw(2000)
except InsufficientFundsError as e:
    print(f"Error: {e}")
finally:
    print(f"Current balance: {acc.balance}")

# Task 2: safe file read
try:
    with open("notes.txt", "r") as f:
        print(f.read())
except FileNotFoundError:
    print("File not found, please check the filename.")
finally:
    print("Read attempt finished")

# Task 3: dict comprehension + sort
words = ["python", "is", "fun", "and", "powerful"]
word_lengths = {w: len(w) for w in words}
sorted_words = dict(sorted(word_lengths.items(), key=lambda item: item[1]))
print(sorted_words)
You should see
If you solve every task correctly, the output will show the withdraw error message plus the balance, the file-read result (or a friendly error), and a dictionary sorted by length.

Try It: 5 Minutes

Set a 10-minute timer and write the BankAccount class from Task 1 yourself - try customizing the custom exception message with your own text.

A Quick Warning

When creating custom exceptions, be careful not to reuse the names of built-in exceptions. And remember, the finally block is always worth using for resource cleanup, like closing files or connections.

Easy traps

  • Writing a custom exception class without inheriting from Exception, which breaks both the raise and the except
  • Putting with open() outside the try block, so FileNotFoundError never actually gets caught by except

Now Try It Yourself

Set a 10-minute timer and write the BankAccount class from Task 1 yourself - try customizing the custom exception message with your own text.

You'll know it worked when: If you solve every task correctly, the output will show the withdraw error message plus the balance, the file-read result (or a friendly error), and a dictionary sorted by length.

Exercise Set 2: Intermediate Practice (OOP, Files, Errors, Comprehensions) | Thuta Learning