What is the proper way to declare custom exceptions in modern Python?

To declare a custom exception in modern Python (3.x), you need to create a new class that inherits from the built-in Exception class. Here's an example:

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

To raise the custom exception, use the raise keyword, followed by an instance of the exception class:

raise MyCustomException("Something went wrong")

try:
    # Some code here that might raise the custom exception
except MyCustomException as e:
    print(e.message)