43. Creating Custom Python Exceptions

Here are 10 Python code snippets demonstrating how to create custom exceptions, which allow for more specialized and accurate error handling:

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1. Basic Custom Exception

Define a basic custom exception by subclassing Exception.

class CustomError(Exception):
    pass

raise CustomError("This is a custom error.")

This creates a basic custom exception called CustomError.

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2. Custom Exception with Error Message

You can add a custom message to your exception.

class InvalidAgeError(Exception):
    def __init__(self, message="Age must be between 0 and 120"):
        self.message = message
        super().__init__(self.message)

raise InvalidAgeError("Invalid age provided.")

This exception class includes an initialization method that accepts a custom error message.

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3. Custom Exception with Additional Attributes

Custom exceptions can store additional data for better context.

class DatabaseConnectionError(Exception):
    def __init__(self, db_name, message="Failed to connect to the database"):
        self.db_name = db_name
        self.message = message
        super().__init__(f"{self.message}: {self.db_name}")

raise DatabaseConnectionError("my_database")

This custom exception includes the database name along with the error message.

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4. Custom Exception with Logging

You can create an exception that logs the error when raised.

import logging

class LoggingError(Exception):
    def __init__(self, message):
        self.message = message
        logging.error(message)
        super().__init__(self.message)

raise LoggingError("Something went wrong!")

This logs the error message when the exception is raised.

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5. Custom Exception with Custom String Representation

Override the __str__ method to customize the exception string.

class DivisionByZeroError(Exception):
    def __init__(self, num):
        self.num = num

    def __str__(self):
        return f"Cannot divide {self.num} by zero."

raise DivisionByZeroError(10)

This custom exception provides a specialized string representation when printed.

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6. Custom Exception with __repr__ for Debugging

Override the __repr__ method for better debugging output.

class InsufficientFundsError(Exception):
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount

    def __repr__(self):
        return f"InsufficientFundsError(balance={self.balance}, amount={self.amount})"

raise InsufficientFundsError(100, 200)

The __repr__ method provides a more detailed output for debugging purposes.

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7. Custom Exception with Multiple Constructors

Create custom exceptions with multiple ways to initialize.

class FileReadError(Exception):
    def __init__(self, filename=None, message="Error reading file"):
        self.filename = filename
        self.message = message
        super().__init__(self.message)

    @classmethod
    def from_file(cls, filename):
        return cls(filename, f"Error reading file {filename}")

raise FileReadError.from_file("myfile.txt")

This example allows creating the exception either from a filename or with a default message.

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8. Custom Exception for Specific Use Case (Input Validation)

A custom exception can be designed for input validation scenarios.

class InvalidInputError(ValueError):
    def __init__(self, input_value, message="Invalid input value"):
        self.input_value = input_value
        self.message = message
        super().__init__(f"{self.message}: {self.input_value}")

raise InvalidInputError("123abc")

This custom exception inherits from ValueError and adds a specific error message for invalid inputs.

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9. Custom Exception for Resource Not Found

Design a custom exception for when a resource is not found.

class ResourceNotFoundError(Exception):
    def __init__(self, resource_name):
        self.resource_name = resource_name
        self.message = f"Resource '{self.resource_name}' not found"
        super().__init__(self.message)

raise ResourceNotFoundError("User data")

This exception could be raised when a resource (e.g., a file, database record) cannot be found.

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10. Custom Exception with a Stack Trace

Capture the stack trace when raising a custom exception for debugging purposes.

import traceback

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

    def log_stack_trace(self):
        return traceback.format_exc()

try:
    raise CustomStackTraceError("An error occurred")
except CustomStackTraceError as e:
    print(f"Error: {e}")
    print(f"Stack trace: {e.log_stack_trace()}")

This custom exception logs the stack trace when it’s raised.

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These custom exception examples can be used to add more specific, helpful, and organized error handling in your Python applications, making the debugging and error logging process more robust.