Why custom exceptions are needed in Python - See It in Action

Why Custom Exceptions Are Needed

📖 Scenario: Imagine you are building a simple banking app. You want to handle errors like trying to withdraw more money than the account has. Python has built-in errors, but sometimes you want to create your own special error messages that make it clear what went wrong in your app.

🎯 Goal: You will create a custom exception called InsufficientFundsError to show a clear message when someone tries to take out too much money from their bank account.

📋 What You'll Learn

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

Create a variable balance with the value 100.

Create a variable withdraw_amount with the value 150.

Write an if statement to check if withdraw_amount is greater than balance.

If it is, raise the InsufficientFundsError with the message 'Not enough money in your account.'.

If not, subtract withdraw_amount from balance.

Print the remaining balance.

💡 Why This Matters

🌍 Real World

Custom exceptions are used in real apps to give clear error messages that help users and developers understand problems quickly.

💼 Career

Knowing how to create and use custom exceptions is important for writing clean, maintainable code in software development jobs.

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 inside write pass.

2

Set up the 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

Write balance = 100 and withdraw_amount = 150.

3

Check if withdrawal is possible and raise exception

Write an if statement to check if withdraw_amount is greater than balance. If it is, raise the InsufficientFundsError with the message 'Not enough money in your account.'. Otherwise, subtract withdraw_amount from balance.

Python

class InsufficientFundsError(Exception):
    pass

balance = 100
withdraw_amount = 150

# Your code here

Hint

Use if withdraw_amount > balance: then raise InsufficientFundsError('Not enough money in your account.'). Else subtract.

4

Print the remaining balance

Write print(balance) to display the remaining balance after the withdrawal check.

Python

class InsufficientFundsError(Exception):
    pass

balance = 100
withdraw_amount = 150

if withdraw_amount > balance:
    raise InsufficientFundsError('Not enough money in your account.')
else:
    balance -= withdraw_amount

# Your code here

Hint

Just write print(balance). But since withdrawal is too big, the program will raise the custom error instead of printing.

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