121. Custom Exception Handling

Here are 10 Python code snippets demonstrating how to create and use custom exceptions for specific error scenarios:

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1. Basic Custom Exception

class CustomError(Exception):
    """Base class for other exceptions"""
    pass

def check_value(value):
    if value < 0:
        raise CustomError("Value cannot be negative")

try:
    check_value(-1)
except CustomError as e:
    print(f"Error: {e}")

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2. Custom Exception with Message

class ValueTooLowError(Exception):
    def __init__(self, message="Value is too low"):
        self.message = message
        super().__init__(self.message)

def process_value(value):
    if value < 10:
        raise ValueTooLowError("Value must be greater than or equal to 10")
    return value * 2

try:
    result = process_value(5)
except ValueTooLowError as e:
    print(f"Error: {e}")

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3. Custom Exception with Additional Attributes

class InvalidInputError(Exception):
    def __init__(self, value, message="Invalid input provided"):
        self.value = value
        self.message = message
        super().__init__(self.message)

def validate_input(value):
    if value < 0:
        raise InvalidInputError(value, "Negative value is not allowed")
    return True

try:
    validate_input(-5)
except InvalidInputError as e:
    print(f"Error: {e.message} (Value: {e.value})")

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4. Custom Exception with Traceback

import traceback

class CustomExceptionWithTraceback(Exception):
    pass

def error_function():
    try:
        raise CustomExceptionWithTraceback("Something went wrong!")
    except CustomExceptionWithTraceback as e:
        traceback.print_exc()  # Print the traceback

error_function()

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5. Catching Multiple Custom Exceptions

class NegativeValueError(Exception):
    pass

class ZeroValueError(Exception):
    pass

def validate_number(value):
    if value < 0:
        raise NegativeValueError("Negative value is not allowed")
    elif value == 0:
        raise ZeroValueError("Zero value is not allowed")

try:
    validate_number(-10)
except (NegativeValueError, ZeroValueError) as e:
    print(f"Error: {e}")

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6. Re-Raising Custom Exceptions

class CustomFileError(Exception):
    pass

def open_file(file_name):
    try:
        with open(file_name, 'r') as file:
            return file.read()
    except FileNotFoundError:
        raise CustomFileError(f"The file {file_name} was not found.")

try:
    content = open_file("non_existing_file.txt")
except CustomFileError as e:
    print(f"Custom error: {e}")

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7. Custom Exception with Context Manager

class CustomContextManagerError(Exception):
    pass

class CustomContextManager:
    def __enter__(self):
        print("Entering the context...")
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type:
            print(f"An error occurred: {exc_value}")
            raise CustomContextManagerError("Custom exception during context exit.")
        print("Exiting the context...")

try:
    with CustomContextManager() as cm:
        raise ValueError("A value error occurred!")
except CustomContextManagerError as e:
    print(f"Caught custom exception: {e}")

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8. Custom Exception with String Representation

class AgeNotValidError(Exception):
    def __init__(self, age, message="Age is not valid"):
        self.age = age
        self.message = message
        super().__init__(self.message)

    def __str__(self):
        return f"{self.message}: {self.age}"

def check_age(age):
    if age < 18:
        raise AgeNotValidError(age, "Age must be at least 18")

try:
    check_age(16)
except AgeNotValidError as e:
    print(f"Error: {e}")

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9. Custom Exception for Invalid Operation

class InvalidOperationError(Exception):
    def __init__(self, operation, message="Invalid operation attempted"):
        self.operation = operation
        self.message = message
        super().__init__(self.message)

def perform_operation(a, b, operation):
    if operation == "divide" and b == 0:
        raise InvalidOperationError(operation, "Cannot divide by zero")
    return a / b if operation == "divide" else a + b

try:
    result = perform_operation(10, 0, "divide")
except InvalidOperationError as e:
    print(f"Error: {e.message} during {e.operation} operation")

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10. Custom Exception for Database Error

class DatabaseConnectionError(Exception):
    def __init__(self, database, message="Could not connect to the database"):
        self.database = database
        self.message = message
        super().__init__(self.message)

def connect_to_database(db_name):
    if db_name != "mydb":
        raise DatabaseConnectionError(db_name, f"Unable to connect to {db_name}")
    return "Connected to database"

try:
    connect_to_database("otherdb")
except DatabaseConnectionError as e:
    print(f"Error: {e.message} ({e.database})")

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These snippets showcase different ways to create, use, and extend custom exceptions in Python to handle specific error scenarios effectively.