Best practices for custom exceptions in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

Why should you create custom exceptions in Python?

Custom exceptions help you handle specific errors clearly and make your code easier to understand and debug.

beginner

What is the recommended base class for custom exceptions in Python?

Custom exceptions should inherit from the built-in Exception class, not from BaseException.

beginner

How should you name custom exception classes?

Use clear, descriptive names ending with Error to show they represent errors, like ValidationError.

beginner

Why is it important to add a useful message to your custom exceptions?

A clear message helps users and developers understand what went wrong and how to fix it.

intermediate

Should custom exceptions include additional data or methods?

Yes, if needed. Adding extra information or helper methods can make error handling more effective and informative.

What is the best base class to inherit from when creating a custom exception?

A Error

B BaseException

C Exception

D CustomError

Custom exceptions should inherit from Exception to avoid catching system-exiting exceptions like KeyboardInterrupt.

Which of the following is a good name for a custom exception?

A DataExceptionClass

B DataError

C ErrorData

D ExceptionData

Names ending with 'Error' clearly indicate an exception, making 'DataError' a good choice.

Why should custom exceptions include a message?

A To provide clear information about the error

B To confuse users

C To make the code longer

D To avoid catching exceptions

A clear message helps understand the cause of the error and how to fix it.

Which practice is NOT recommended when creating custom exceptions?

A Inheriting from Exception

B Adding helpful error messages

C Including extra data if useful

D Using vague names like 'Error1'

Vague names make it hard to understand the error; descriptive names are better.

What should you do if your custom exception needs to carry extra information?

A Add attributes or methods to the exception class

B Ignore it

C Raise a different exception

D Use print statements

Adding attributes or methods helps provide more context for the error.

Explain why and how you would create a custom exception in Python.

Think about making your error handling clearer and easier to debug.

List best practices to follow when designing custom exceptions.

Focus on clarity, usability, and helpful information.

Practice

1. Why is it a good practice to create custom exceptions in Python?

easy

A. To make error handling clearer and more specific

B. To avoid using try-except blocks

C. To speed up the program execution

D. To replace all built-in exceptions

Solution

1. Step 1: Understand the purpose of custom exceptions

   Custom exceptions help programmers identify and handle specific errors clearly.

2. Step 2: Compare with other options

   Options B, C, and D are incorrect because custom exceptions do not avoid try-except, speed execution, or replace built-ins.

3. Final Answer:

   To make error handling clearer and more specific -> Option A

4. Quick Check:

   Custom exceptions clarify errors = A [OK]

Hint: Custom exceptions clarify specific errors [OK]

Common Mistakes:

• Thinking custom exceptions speed up code
• Believing they replace built-in exceptions
• Assuming they remove need for try-except

2. Which of the following is the correct way to define a custom exception in Python?

easy

A. class MyError(Exception): pass

B. def MyError(): pass

C. class MyError: pass

D. exception MyError(Exception): pass

Solution

1. Step 1: Check Python syntax for custom exceptions

   Custom exceptions must be classes inheriting from Exception or its subclasses.

2. Step 2: Evaluate each option

   A defines a class without inheritance, B defines a function, D uses invalid keyword. Only C ('class MyError(Exception): pass') correctly defines a custom exception.

3. Final Answer:

   class MyError(Exception): pass -> Option A

4. Quick Check:

   Custom exceptions are classes inheriting Exception = C [OK]

Hint: Custom exceptions are classes inheriting Exception [OK]

Common Mistakes:

• Defining exceptions as functions
• Not inheriting from Exception
• Using invalid keywords like 'exception'

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. No output

B. MyError

C. Exception

D. Oops!

Solution

1. Step 1: Understand the raise statement

   The code raises MyError with message 'Oops!'.

2. Step 2: Check the except block output

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

3. Final Answer:

   Oops! -> Option D

4. Quick Check:

   Exception message prints = A [OK]

Hint: Exception message prints when caught and printed [OK]

Common Mistakes:

• Printing exception class name instead of message
• Expecting no output
• Confusing exception type with message

4. Identify the error in this custom exception definition:

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

raise CustomError('Error occurred')

medium

A. No __str__ method to display message

B. CustomError does not inherit from Exception

C. Message attribute should be named 'msg'

D. Missing parentheses in raise statement

Solution

1. Step 1: Check inheritance of CustomError

   CustomError does not inherit from Exception, so it is not a proper exception class.

2. Step 2: Analyze other options

   Raise syntax is correct, attribute name is flexible, __str__ is optional but recommended.

3. Final Answer:

   CustomError does not inherit from Exception -> Option B

4. Quick Check:

   Custom exceptions must inherit Exception = D [OK]

Hint: Always inherit Exception for custom exceptions [OK]

Common Mistakes:

• Forgetting to inherit from Exception
• Assuming attribute names must be fixed
• Thinking __str__ is mandatory

5. You want to create a custom exception that stores an error code and a message. Which is the best practice to implement it?

hard

A. class ErrorCodeException(Exception):
def __init__(self, code, message):
self.code = code
self.message = message

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

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

D. class ErrorCodeException(Exception):
pass

Solution

1. Step 1: Check inheritance and initialization

   class ErrorCodeException(Exception): def __init__(self, code, message): super().__init__(message) self.code = code inherits Exception and calls super().__init__(message) to set the message properly.

2. Step 2: Compare with other options

   class ErrorCodeException(Exception): def __init__(self, code, message): self.code = code self.message = message does not call super().__init__, so message may not behave like a normal exception message. class ErrorCodeException: def __init__(self, code, message): self.code = code self.message = message lacks inheritance. class ErrorCodeException(Exception): pass has no code or message storage.

3. Final Answer:

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

4. Quick Check:

   Inherit Exception and call super() with message = B [OK]

Hint: Call super().__init__(message) to set exception message [OK]

Common Mistakes:

• Not calling super().__init__ for message
• Not inheriting from Exception
• Storing message without Exception support