Why custom exceptions are needed in Python - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to raise a built-in exception when a number is negative.

Python

if number < 0:
    raise [1]("Negative number not allowed")

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

AValueError

BCustomError

CException

DTypeError

Attempts:

3 left

💡 Hint

Common Mistakes

Using a custom exception when a built-in one fits.

Using TypeError which is for wrong data types.

ValueError is the built-in exception used to indicate a wrong value, such as a negative number where it is not allowed.

2fill in blank

medium

Complete the code to define a custom exception named MyError.

Python

class [1](Exception):
    pass

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

AError

BMyError

CCustomException

DException

Attempts:

3 left

💡 Hint

Common Mistakes

Using Exception as the class name.

Not inheriting from Exception.

Custom exceptions are defined by creating a new class that inherits from Exception. Here, MyError is the custom exception name.

3fill in blank

hard

Fix the error in raising the custom exception with a message.

Python

raise [1]("This is a custom error message")

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

AError

BCustomError

CMyError

DException

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic Exception instead of the custom one.

Using a wrong class name.

To raise the custom exception, use its class name MyError, which was defined earlier.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that includes only keys with values greater than 10.

Python

filtered = {k: v for k, v in data.items() if v [1] 10 and isinstance(v, [2])}

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

A>

Bint

C<

Dstr

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for comparison.

Checking for str instead of int.

The condition filters values greater than 10 and checks if the value is an integer.

5fill in blank

hard

Fill all three blanks to create a custom exception class with a message attribute and raise it.

Python

class [1](Exception):
    def __init__(self, message):
        self.[2] = message
        super().__init__(message)

raise [3]("Custom error occurred")

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

ACustomError

Bmsg

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for class and raise.

Using 'msg' instead of 'message' attribute.

The class name is CustomError. The attribute to store the message is 'message'. The exception is raised using the class name CustomError.

Practice

1. Why do programmers create custom exceptions instead of using only built-in exceptions?

easy

A. To clearly identify and handle specific errors unique to their program

B. Because built-in exceptions are slower to execute

C. To avoid writing any error handling code

D. Because Python does not have any built-in exceptions

Solution

1. Step 1: Understand the purpose of exceptions

   Exceptions help handle errors during program execution. Built-in exceptions cover common errors.

2. Step 2: Recognize the need for custom exceptions

   Custom exceptions let programmers mark and handle errors specific to their program clearly and separately.

3. Final Answer:

   To clearly identify and handle specific errors unique to their program -> Option A

4. Quick Check:

   Custom exceptions = specific error handling [OK]

Hint: Custom exceptions clarify unique errors in your code [OK]

Common Mistakes:

• Thinking built-in exceptions are slow
• Believing custom exceptions remove need for error handling
• Assuming Python lacks built-in exceptions

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

easy

A. def MyError(): pass

B. class MyError(Exception): pass

C. class MyError: pass

D. exception MyError(Exception): pass

Solution

1. Step 1: Recall syntax for custom exceptions

   Custom exceptions are classes that inherit from Exception or its subclasses.

2. Step 2: Identify correct class definition

   class MyError(Exception): pass correctly defines MyError as a subclass of Exception with pass to keep it simple.

3. Final Answer:

   class MyError(Exception): pass -> Option B

4. Quick Check:

   Custom exception = class inheriting Exception [OK]

Hint: Custom exceptions are classes inheriting Exception [OK]

Common Mistakes:

• Defining exception as a function
• Not inheriting from Exception
• Using wrong keyword like 'exception'

3. What will be the output of this code?

class MyError(Exception):
    pass

def test(value):
    if value < 0:
        raise MyError("Negative value")
    return value

try:
    print(test(-1))
except MyError as e:
    print(e)

medium

A. Negative value

B. -1

C. None

D. No output

Solution

1. Step 1: Analyze function behavior

   The function test raises MyError with message "Negative value" if input is less than 0.

2. Step 2: Trace try-except block

   Calling test(-1) raises MyError. The except block catches it and prints the error message.

3. Final Answer:

   Negative value -> Option A

4. Quick Check:

   Raised custom exception message printed [OK]

Hint: Raised custom exception prints its message in except block [OK]

Common Mistakes:

• Expecting function to return -1
• Thinking no output occurs
• Confusing exception name with message

4. Find the error in this custom exception usage:

class MyError(Exception):
    pass

try:
    raise MyError("Oops")
except Exception as e:
    print("Error:", e.message)

medium

A. Custom exception must not inherit Exception

B. except block should catch MyError, not Exception

C. raise keyword is missing

D. Using e.message to get error text causes AttributeError

Solution

1. Step 1: Check exception message access

   In Python, exception objects do not have a message attribute by default.

2. Step 2: Identify correct way to get message

   The message is accessed by converting the exception to string or using args. Using e.message causes an AttributeError.

3. Final Answer:

   Using e.message to get error text causes AttributeError -> Option D

4. Quick Check:

   Exception message accessed via str(e), not e.message [OK]

Hint: Use str(e) to get exception message, not e.message [OK]

Common Mistakes:

• Assuming e.message exists
• Thinking custom exceptions can't inherit Exception
• Missing raise keyword
• Believing except must catch only MyError

5. You want to create a custom exception InvalidAgeError that triggers when age is below 0 or above 120. Which approach best uses custom exceptions to handle this validation?

hard

A. Print error message instead of raising exceptions

B. Use only built-in ValueError without custom exceptions

C. Define InvalidAgeError inheriting Exception, raise it in a function checking age limits

D. Catch all exceptions with a generic except block without custom exceptions

Solution

1. Step 1: Understand validation needs

   Age must be checked for invalid values and a clear error raised if invalid.

2. Step 2: Use custom exception for clarity

   Defining InvalidAgeError inheriting from Exception and raising it on invalid age clearly signals this specific error.

3. Step 3: Compare other options

   Using only built-in exceptions or printing errors reduces clarity and control. Catching all exceptions generically hides specific issues.

4. Final Answer:

   Define InvalidAgeError inheriting Exception, raise it in a function checking age limits -> Option C

5. Quick Check:

   Custom exception for specific validation error [OK]

Hint: Raise custom exceptions for clear, specific validation errors [OK]

Common Mistakes:

• Relying only on built-in exceptions
• Printing errors instead of raising
• Using generic except blocks hiding issues