Creating exception classes in Python - Try It Yourself

Creating exception classes

📖 Scenario: Imagine you are building a simple banking system. You want to handle errors like when someone tries to withdraw more money than they have.

🎯 Goal: You will create a custom exception class called InsufficientFundsError to handle this specific error. Then you will use it in a function that withdraws money from an account.

📋 What You'll Learn

Create a custom exception class named InsufficientFundsError that inherits from Exception.

Create a variable balance with the value 100.

Create a variable withdraw_amount with the value 150.

Write a function withdraw that takes amount as a parameter.

Inside the function, raise InsufficientFundsError with the message 'Not enough money in the account' if amount is greater than balance.

Call the withdraw function with withdraw_amount inside a try block.

Catch the InsufficientFundsError exception and print its message.

💡 Why This Matters

🌍 Real World

Custom exceptions help you handle specific errors in your programs clearly, like when a bank account has insufficient funds.

💼 Career

Knowing how to create and use custom exceptions is important for writing robust software that handles errors gracefully in real-world applications.

1

Create the custom exception class

Create a custom exception class called InsufficientFundsError that inherits from Exception.

Python

# Your code here

Hint

Use class InsufficientFundsError(Exception): and add pass inside.

2

Set up balance and withdraw amount

Create a variable called balance and set it to 100. Then create a variable called withdraw_amount and set it to 150.

Python

class InsufficientFundsError(Exception):
    pass

# Your code here

Hint

Use simple assignment like balance = 100 and withdraw_amount = 150.

3

Write the withdraw function with exception

Write a function called withdraw that takes a parameter amount. Inside the function, if amount is greater than balance, raise InsufficientFundsError with the message 'Not enough money in the account'.

Python

class InsufficientFundsError(Exception):
    pass

balance = 100
withdraw_amount = 150

# Your code here

Hint

Use def withdraw(amount): and inside check if amount > balance: then raise InsufficientFundsError('Not enough money in the account').

4

Call withdraw and handle exception

Call the withdraw function with withdraw_amount inside a try block. Catch the InsufficientFundsError exception and print its message.

Python

class InsufficientFundsError(Exception):
    pass

balance = 100
withdraw_amount = 150

def withdraw(amount):
    if amount > balance:
        raise InsufficientFundsError('Not enough money in the account')

# Your code here

Hint

Use try: to call withdraw(withdraw_amount) and except InsufficientFundsError as e: to print e.

Practice

1. What is the correct way to create a custom exception class in Python?

easy

A. exception MyError(Exception): pass

B. def MyError(): raise Exception

C. class MyError(Exception): pass

D. class MyError: pass

Solution

1. Step 1: Understand how to define a class inheriting Exception

   Custom exceptions must inherit from the built-in Exception class to behave like errors.

2. Step 2: Check syntax correctness

   class MyError(Exception): pass correctly defines a class named MyError inheriting from Exception with pass inside.

3. Final Answer:

   class MyError(Exception): pass -> Option C

4. Quick Check:

   Custom exception class = class MyError(Exception): pass [OK]

Hint: Inherit from Exception to create custom errors [OK]

Common Mistakes:

• Not inheriting from Exception
• Using def instead of class
• Wrong keyword like 'exception' instead of 'class'

2. Which of the following is the correct syntax to raise a custom exception named MyError?

easy

A. raise MyError()

B. throw MyError()

C. raise new MyError()

D. throw new MyError()

Solution

1. Step 1: Recall the Python keyword to raise exceptions

   Python uses the keyword 'raise' to trigger exceptions, not 'throw'.

2. Step 2: Check the syntax for raising a custom exception

   Correct syntax is 'raise MyError()' to create and raise the exception instance.

3. Final Answer:

   raise MyError() -> Option A

4. Quick Check:

   Raise custom error = raise MyError() [OK]

Hint: Use 'raise' keyword followed by exception instance [OK]

Common Mistakes:

• Using 'throw' instead of 'raise'
• Adding 'new' keyword like in other languages
• Not calling the exception as a function

3. What will be the output of this code?

class MyError(Exception):
    pass

try:
    raise MyError("Oops!")
except MyError as e:
    print(e)

medium

A. Oops!

B. MyError

C. Exception

D. No output

Solution

1. Step 1: Understand the raise statement with message

   The code raises MyError with the message 'Oops!'.

2. Step 2: Catch the exception and print its message

   The except block catches MyError as 'e' and prints 'e', which outputs the message 'Oops!'.

3. Final Answer:

   Oops! -> Option A

4. Quick Check:

   Exception message prints = Oops! [OK]

Hint: Exception instance prints its message string [OK]

Common Mistakes:

• Printing exception class name instead of message
• Not catching the exception properly
• Expecting no output

4. Identify the error in this code snippet:

class MyError(Exception):
    pass

try:
    raise MyError
except MyError:
    print("Caught error")

medium

A. Incorrect exception name in except block

B. No error, code runs fine

C. Syntax error in class definition

D. Missing parentheses when raising MyError

Solution

1. Step 1: Check how the exception is raised

   In Python, it is valid to raise an exception class without parentheses if it has no __init__ arguments.

2. Step 2: Identify the problem in the code

   The code uses 'raise MyError' without parentheses, which is valid and does not raise an error.

3. Final Answer:

   No error, code runs fine -> Option B

4. Quick Check:

   Raising exception class without parentheses is allowed [OK]

Hint: Raising exception class without parentheses is valid if no arguments [OK]

Common Mistakes:

• Omitting parentheses after exception name
• Mismatching exception names in except block
• Incorrect class syntax

5. You want to create a custom exception ValidationError that stores an error code along with the message. Which code correctly implements this?

hard

A. class ValidationError(Exception):
def __init__(self, message):
self.code = 0
super().__init__(message)

B. class ValidationError(Exception):
def __init__(self, message, code):
self.message = message
self.code = code

C. class ValidationError(Exception):
def __init__(self, code):
super().__init__(code)

D. class ValidationError(Exception):
def __init__(self, message, code):
super().__init__(message)
self.code = code

Solution

1. Step 1: Understand how to extend Exception with extra attributes

   To add an error code, override __init__ and call super().__init__(message) to set the message properly.

2. Step 2: Check which option correctly calls super().__init__ and stores code

   class ValidationError(Exception): def __init__(self, message, code): super().__init__(message) self.code = code calls super().__init__(message) and assigns self.code = code, correctly storing both.

3. Final Answer:

   class ValidationError(Exception): def __init__(self, message, code): super().__init__(message) self.code = code -> Option D

4. Quick Check:

   Call super().__init__(message) and store extra attributes [OK]

Hint: Call super().__init__(message) to set message, then add code [OK]

Common Mistakes:

• Not calling super().__init__ for message
• Assigning message without super call
• Missing code attribute assignment