Raising Custom Exceptions in Python

This snippet demonstrates how to raise custom exceptions in Python using the raise keyword. It also covers defining custom exception classes that inherit from the base Exception class. Understanding exception handling is crucial for writing robust and maintainable Python code.

Basic Example: Raising a Built-in Exception

This code defines a function check_age that raises a ValueError if the age is negative. The try...except block catches the exception and prints an error message. This illustrates the basic use of the raise keyword with a built-in exception type.

def check_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    else:
        print("Age is valid.")

try:
    check_age(-5)
except ValueError as e:
    print(f"Error: {e}")

check_age(25)

Creating a Custom Exception Class

This example demonstrates how to define a custom exception class called InsufficientFundsError that inherits from the base Exception class. It overrides the __init__ method to accept a custom error message. The withdraw function now raises this custom exception when the withdrawal amount exceeds the balance. The try...except block catches the custom exception.

class InsufficientFundsError(Exception):
    def __init__(self, message):
        self.message = message
        super().__init__(self.message)

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientFundsError("Insufficient funds in your account")
    else:
        print("Withdrawal successful.")
        return balance - amount

balance = 100

try:
    new_balance = withdraw(balance, 150)
    print(f"New balance: {new_balance}")
except InsufficientFundsError as e:
    print(f"Error: {e}")

Explanation of raise keyword

The raise keyword is used to explicitly raise an exception in Python. It allows you to signal that an error or unexpected condition has occurred during the execution of your code. You can raise built-in exceptions or create your own custom exceptions.

Real-Life Use Case Section

Imagine building an e-commerce platform. You might want to raise a ProductOutOfStockError if a customer tries to purchase a product that is no longer available. Similarly, a InvalidCouponCodeError could be raised if a user enters an incorrect coupon code. Raising exceptions allows your code to handle such situations gracefully and inform the user about the problem.

Best Practices

• Be Specific: Raise specific exception types whenever possible. This makes it easier to understand the nature of the error and handle it appropriately.
• Include Helpful Messages: Provide clear and informative error messages to help debug the issue.
• Don't Catch Everything: Avoid catching generic Exception unless you have a very good reason. Catch only the specific exceptions you expect and know how to handle.
• Clean Up Resources: Use try...finally blocks to ensure that resources (like files or network connections) are properly cleaned up, even if an exception occurs.

Interview Tip

During interviews, be prepared to discuss exception handling best practices, including when to use custom exceptions and how to write effective try...except blocks. Demonstrate your understanding of the different types of exceptions and how to handle them gracefully.

When to Use Them

Use exceptions when your code encounters an error condition that it cannot handle itself. This allows the calling code to take appropriate action, such as logging the error, retrying the operation, or informing the user.

Alternatives

While exceptions are the preferred method for handling errors in Python, alternative approaches include returning error codes or using special return values to indicate failure. However, these methods can be less readable and harder to maintain compared to using exceptions, especially for complex error scenarios.

Pros

• Clear Error Handling: Exceptions provide a structured and explicit way to handle errors.
• Improved Code Readability: Error handling logic is separated from the main code flow.
• Easier Debugging: Stack traces provide valuable information for debugging.

Cons

• Performance Overhead: Raising exceptions can have a performance overhead, especially if they are raised frequently.
• Potential for Unhandled Exceptions: If exceptions are not properly handled, they can cause the program to terminate unexpectedly.