Define Custom Exceptions in Python

In Python, exceptions occur during the execution of a program that disrupts the normal flow of the program’s instructions. When an error occurs, Python raises an exception, which can be caught and handled using try and except blocks. Here’s a simple example of handling a built-in exception:

try:
    result = 10 / 0
except ZeroDivisionError:
    print("You cannot divide by zero!")

In this example, ZeroDivisionError is a built-in exception that gets raised when you attempt to divide by zero.

Why Define Custom Exceptions?

Custom exceptions are useful in the following scenarios:

1. Clarity: They provide clear, specific error messages that are relevant to your application.
2. Granularity: They allow for more fine-grained error handling, making it easier to pinpoint and address specific issues.
3. Reusability: They can be reused across different parts of your application or even in different projects.

1. Defining a Custom Exception

To define a custom exception in Python, you need to create a new class that inherits from the built-in Exception class or one of its subclasses. Here’s a basic example:

class MyCustomError(Exception):
    """Exception raised for custom error scenarios.

    Attributes:
        message -- explanation of the error
    """

    def __init__(self, message):
        self.message = message
        super().__init__(self.message)

In this example, MyCustomError is a custom exception class that inherits from Exception. It has an __init__ method that takes a message parameter and passes it to the base class constructor.

2. Defining a Custom Exception

To define a custom exception we create a new class that inherits from the built-in ‘Exception’ class and override its methods to customize its behavior for specificity.

class MyCustomError(Exception):
    """Exception raised for custom error in the application."""

    def __init__(self, message, error_code):
        super().__init__(message)
        self.error_code = error_code

    def __str__(self):
        return f"{self.message} (Error Code: {self.error_code})"

3. Raising a Custom Exception

To raise a custom exception, use the raise keyword followed by an instance of your custom exception.

def divide(a, b):
    if b == 0:
        raise MyCustomError("Division by zero is not allowed", 400)
    return a / b

Here the divide method raises the ‘MyCustomError’ when an attempt to divide by zero is made.

4. Handling Custom Exceptions

Custom exceptions can be handled similar to built-in exceptions using a `try…except` block.

try:
    result = divide(10, 0)
except MyCustomError as e:
    print(f"Caught an error: {e}")

Here divide function raises `MyCustomError` and it is caught and handled by the `except` block. Additional attributes and methods can be used to enhance Custom Exceptions to provide more context or functionality.

class FileProcessingError(Exception):
    def __init__(self, message, filename, lineno):
        super().__init__(message)
        self.filename = filename
        self.lineno = lineno

    def __str__(self):
        return f"{self.message} in {self.filename} at line {self.lineno}"


try:
    raise FileProcessingError("Syntax error", "example.txt", 13)
except FileProcessingError as e:
    print(f"Caught an error: {e}")

Tips for Effective Use of Custom Exceptions

When defining and using custom exceptions, consider the following tips to ensure they are effective and maintainable:

• Naming: Use clear and descriptive names that end with Error to indicate that they are exceptions.
• Documentation: Provide docstrings for your custom exceptions to explain when they should be used.
• Hierarchy: Create a hierarchy of exceptions if your application requires handling different types of errors differently python

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