Best practices for custom exceptions in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom exception handling

What will be the output of this Python code that defines and uses a custom exception?

Python

class MyError(Exception):
    pass

def test():
    try:
        raise MyError("Oops")
    except MyError as e:
        print(f"Caught: {e}")

try:
    test()
except Exception:
    print("General exception caught")

A Caught: Oops

B General exception caught

C No output

D SyntaxError

💡 Hint

Look at which exception is raised and which is caught inside the test function.

The custom exception MyError is raised and caught inside the test function's try-except block, so it prints 'Caught: Oops'. The outer try-except does not run its except block.

🧠 Conceptual

intermediate

Why inherit from Exception for custom exceptions?

Why is it recommended to inherit custom exceptions from the built-in Exception class in Python?

A Because inheriting from Exception allows the custom exception to be caught by standard except blocks.

B Because inheriting from Exception disables the traceback.

C Because inheriting from Exception automatically logs the error.

D Because inheriting from Exception makes the exception a syntax error.

💡 Hint

Think about how Python's try-except blocks catch exceptions.

Inheriting from Exception ensures that the custom exception behaves like standard exceptions and can be caught by except Exception blocks.

❓ Predict Output

advanced

Output of custom exception with additional attributes

What is the output of this code that raises a custom exception with extra data?

Python

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

try:
    raise ValidationError("Invalid input", 400)
except ValidationError as e:
    print(f"Error: {e}, Code: {e.code}")

A Error: Invalid input, Code: None

B AttributeError

C Error: ValidationError, Code: 400

D Error: Invalid input, Code: 400

💡 Hint

Check how the message and code are passed and accessed.

The message is passed to the base Exception class and stored as the exception message. The code attribute is stored separately and accessed correctly.

🔧 Debug

advanced

Identify the error in this custom exception definition

What error will this code raise when defining a custom exception?

Python

class MyException:
    def __init__(self, message):
        self.message = message

raise MyException("Error happened")

A NameError

B SyntaxError

C TypeError: exceptions must derive from BaseException

D No error, runs fine

💡 Hint

Check what Python requires for something to be raised as an exception.

Python requires exceptions to inherit from BaseException or its subclasses. This class does not inherit from Exception, so raising it causes a TypeError.

🚀 Application

expert

How many items in the dictionary of custom exceptions?

Given this code, how many keys are in the dictionary 'errors' after execution?

Python

class ErrorA(Exception): pass
class ErrorB(Exception): pass
class ErrorC(ErrorA): pass

errors = {}
for err in [ErrorA, ErrorB, ErrorC]:
    errors[err.__name__] = err

errors['ErrorA'] = 'Custom message'

A 2

B 3

C 1

D 4

💡 Hint

Consider how dictionary keys are assigned and overwritten.

The dictionary initially has 3 keys: 'ErrorA', 'ErrorB', 'ErrorC'. The line errors['ErrorA'] = 'Custom message' replaces the value for 'ErrorA' but does not remove the key. So total keys remain 3.