How to create a custom exception type in Python?

To create a custom exception, you need to inherit from the base class Exception (or one of its subclasses).

# Defining a custom exception
class MyCustomError(Exception):
    # Override the __init__ method to add custom data
    def __init__(self, message, error_code):
        # Call the parent class constructor
        super().__init__(message)
        # Add custom attributes
        self.error_code = error_code

    # Override the __str__ method for a nice output
    def __str__(self):
        return f"Error #{self.error_code}: {self.args[0]}"

Example usage:

# Function that may raise our custom exception
def process_data(data):
    if not isinstance(data, int):
        raise MyCustomError("Invalid data type", 100)
    if data < 0:
        raise MyCustomError("Number must be be positive", 200)
    return data * 2

# Handling the exception
try:
    result = process_data("hello")
    print(result)
except MyCustomError as e:
    print(f"Caught custom exception: {e}")
    print(f"Error code: {e.error_code}")

try:
    result = process_data(-5)
    print(result)
except MyCustomError as e:
    print(f"Caught custom exception: {e}")
    print(f"Error code: {e.error_code}")

try:
    result = process_data(10)
    print(result)
except MyCustomError as e:
    print(f"Caught custom exception: {e}")
    print(f"Error code: {e.error_code}")