Creating exception classes in Python

Introduction

Sometimes, you want to make your own special errors to explain problems clearly in your program.

When you want to show a specific problem that normal errors don't explain well.

When you want to catch and handle a certain type of error differently.

When you want to add extra information to an error to help fix it.

When you want your program to be easier to understand and maintain.

Syntax

Python

class MyError(Exception):
    pass

You create a new error by making a class that uses Exception as its base.

The pass means the new error acts like a normal error unless you add more code.

Examples

This makes a simple new error called MyError.

Python

class MyError(Exception):
    pass

This error stores a message and shows it when printed.

Python

class ValueTooSmallError(Exception):
    def __init__(self, message):
        self.message = message
    def __str__(self):
        return f"ValueTooSmallError: {self.message}"

This error remembers the wrong number and explains why it is wrong.

Python

class NegativeNumberError(Exception):
    def __init__(self, number):
        self.number = number
    def __str__(self):
        return f"NegativeNumberError: {self.number} is not allowed"

Sample Program

This program makes a new error called MyError. It checks if a number is negative. If yes, it raises the error with a message. The try block runs the check. If the error happens, the except block catches it and prints the message.

Python

class MyError(Exception):
    pass

def check_number(x):
    if x < 0:
        raise MyError("Negative numbers are not allowed")
    else:
        print(f"{x} is okay")

try:
    check_number(5)
    check_number(-3)
except MyError as e:
    print(f"Caught an error: {e}")

Important Notes

You can add extra details to your error by adding methods or properties.

Use raise to create your error when a problem happens.

Catch your error with except YourErrorName as e to handle it nicely.

Summary

Create your own errors by making classes that inherit from Exception.

Use raise to trigger your custom error when needed.

Catch and handle your custom errors with try-except blocks.

Practice

1. What is the correct way to create a custom exception class in Python?

easy

A. exception MyError(Exception): pass

B. def MyError(): raise Exception

C. class MyError(Exception): pass

D. class MyError: pass

Solution

1. Step 1: Understand how to define a class inheriting Exception

   Custom exceptions must inherit from the built-in Exception class to behave like errors.

2. Step 2: Check syntax correctness

   class MyError(Exception): pass correctly defines a class named MyError inheriting from Exception with pass inside.

3. Final Answer:

   class MyError(Exception): pass -> Option C

4. Quick Check:

   Custom exception class = class MyError(Exception): pass [OK]

Hint: Inherit from Exception to create custom errors [OK]

Common Mistakes:

• Not inheriting from Exception
• Using def instead of class
• Wrong keyword like 'exception' instead of 'class'

2. Which of the following is the correct syntax to raise a custom exception named MyError?

easy

A. raise MyError()

B. throw MyError()

C. raise new MyError()

D. throw new MyError()

Solution

1. Step 1: Recall the Python keyword to raise exceptions

   Python uses the keyword 'raise' to trigger exceptions, not 'throw'.

2. Step 2: Check the syntax for raising a custom exception

   Correct syntax is 'raise MyError()' to create and raise the exception instance.

3. Final Answer:

   raise MyError() -> Option A

4. Quick Check:

   Raise custom error = raise MyError() [OK]

Hint: Use 'raise' keyword followed by exception instance [OK]

Common Mistakes:

• Using 'throw' instead of 'raise'
• Adding 'new' keyword like in other languages
• Not calling the exception as a function

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. Oops!

B. MyError

C. Exception

D. No output

Solution

1. Step 1: Understand the raise statement with message

   The code raises MyError with the message 'Oops!'.

2. Step 2: Catch the exception and print its message

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

3. Final Answer:

   Oops! -> Option A

4. Quick Check:

   Exception message prints = Oops! [OK]

Hint: Exception instance prints its message string [OK]

Common Mistakes:

• Printing exception class name instead of message
• Not catching the exception properly
• Expecting no output

4. Identify the error in this code snippet:

class MyError(Exception):
    pass

try:
    raise MyError
except MyError:
    print("Caught error")

medium

A. Incorrect exception name in except block

B. No error, code runs fine

C. Syntax error in class definition

D. Missing parentheses when raising MyError

Solution

1. Step 1: Check how the exception is raised

   In Python, it is valid to raise an exception class without parentheses if it has no __init__ arguments.

2. Step 2: Identify the problem in the code

   The code uses 'raise MyError' without parentheses, which is valid and does not raise an error.

3. Final Answer:

   No error, code runs fine -> Option B

4. Quick Check:

   Raising exception class without parentheses is allowed [OK]

Hint: Raising exception class without parentheses is valid if no arguments [OK]

Common Mistakes:

• Omitting parentheses after exception name
• Mismatching exception names in except block
• Incorrect class syntax

5. You want to create a custom exception ValidationError that stores an error code along with the message. Which code correctly implements this?

hard

A. class ValidationError(Exception):
def __init__(self, message):
self.code = 0
super().__init__(message)

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

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

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

Solution

1. Step 1: Understand how to extend Exception with extra attributes

   To add an error code, override __init__ and call super().__init__(message) to set the message properly.

2. Step 2: Check which option correctly calls super().__init__ and stores code

   class ValidationError(Exception): def __init__(self, message, code): super().__init__(message) self.code = code calls super().__init__(message) and assigns self.code = code, correctly storing both.

3. Final Answer:

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

4. Quick Check:

   Call super().__init__(message) and store extra attributes [OK]

Hint: Call super().__init__(message) to set message, then add code [OK]

Common Mistakes:

• Not calling super().__init__ for message
• Assigning message without super call
• Missing code attribute assignment