Best practices for custom exceptions in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a custom exception class named MyError.

Python

class MyError([1]):
    pass

Drag options to blanks, or click blank then click option'

ACustomError

BError

CBaseException

DException

Attempts:

3 left

💡 Hint

Common Mistakes

Inheriting from BaseException causes catching issues.

Using a non-existent base class.

Custom exceptions should inherit from Exception to be caught properly.

2fill in blank

medium

Complete the code to raise the custom exception MyError with a message.

Python

raise MyError([1])

Drag options to blanks, or click blank then click option'

AMyError

BError

C"An error occurred"

DException

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the exception class instead of a message.

Not passing any argument.

When raising an exception, pass a string message describing the error.

3fill in blank

hard

Fix the error in the custom exception class to properly initialize the message.

Python

class MyError(Exception):
    def __init__(self, message):
        [1]

Drag options to blanks, or click blank then click option'

Asuper().__init__(message)

Bself.message = message

CException.__init__(self)

Dself.msg = message

Attempts:

3 left

💡 Hint

Common Mistakes

Only setting an attribute without calling the base constructor.

Calling base constructor without message.

Calling super().__init__(message) properly initializes the base Exception with the message.

4fill in blank

hard

Fill both blanks to create a custom exception with a code attribute and proper initialization.

Python

class MyError(Exception):
    def __init__(self, message, code):
        [1]
        [2] = code

Drag options to blanks, or click blank then click option'

Asuper().__init__(message)

Bself.message

Cself.code

DException.__init__(self, message)

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling the base class __init__.

Assigning code to a wrong attribute name.

Use super().__init__(message) to initialize the message and assign self.code to store the code.

5fill in blank

hard

Fill all three blanks to catch the custom exception and print its message and code.

Python

try:
    raise MyError("Failed", 404)
except [1] as e:
    print(e[2])
    print(e[3])

Drag options to blanks, or click blank then click option'

AMyError

B.args[0]

C.code

DException

Attempts:

3 left

💡 Hint

Common Mistakes

Catching the wrong exception type.

Trying to print message directly without args.

Accessing a non-existent attribute.

Catch the MyError exception, print the first argument (message) from args, and print the code attribute.

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