Exceptionally Clear Errors: How to Declare Custom Exceptions in Python

What are Custom Exceptions?

In Python, exceptions are objects that signal errors or unexpected conditions during program execution. They help control the flow of your code and provide informative messages about what went wrong.

Custom exceptions are exceptions that you define yourself to handle specific errors within your application. This allows you to create more granular error handling tailored to your program's needs.

1. class MyCustomException(Exception):
       pass
   

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

3. def check_age(age):
       if age < 18:
           raise MyCustomException("You must be 18 or older to proceed.")
   

Best Practices:

• Organize Exceptions: Consider creating a separate module (e.g., exceptions.py) to store your custom exception classes. This promotes code organization and reusability.
• Exception Hierarchy: You can create a hierarchy of exceptions by subclassing more specific exception classes from more general ones. This allows for more tailored error handling. For example, you might have a FileProcessingError that inherits from a broader ApplicationError.
• Descriptive Messages: Provide clear and meaningful messages in your custom exceptions to aid in debugging and understanding the root cause of the error.

Example:

# exceptions.py
class FileProcessingError(Exception):
    pass

class InvalidFileDataError(FileProcessingError):
    def __init__(self, message):
        super().__init__(message)

def process_file(filename):
    try:
        # Code that might raise exceptions
        with open(filename, 'r') as file:
            # Process file contents
            if not file.readable():
                raise InvalidFileDataError("File is not readable or corrupt.")
    except InvalidFileDataError as e:
        print(f"Error processing file: {e}")
    except FileProcessingError as e:
        print(f"General file processing error: {e}")
    except Exception as e:
        print(f"Unexpected error: {e}")  # Catch any other exceptions

# main.py
from exceptions import process_file

process_file("data.txt")  # Might raise exceptions based on file contents and processing logic

By following these guidelines, you can effectively define custom exceptions in your Python programs, leading to more robust and informative error handling.

exceptions.py:

class FileProcessingError(Exception):
    """Base class for file processing errors."""
    pass

class InvalidFileDataError(FileProcessingError):
    """Exception raised for invalid data encountered in a file."""
    def __init__(self, message):
        super().__init__(message)  # Call base class constructor for proper inheritance

main.py:

from exceptions import process_file

def process_file(filename):
    try:
        # Code that might raise exceptions
        with open(filename, 'r') as file:
            # Process file contents
            if not file.readable():
                raise InvalidFileDataError("File is not readable or corrupt.")
            # Add additional processing logic here, potentially raising other exceptions
            # (e.g., ValueError for invalid data format)
    except InvalidFileDataError as e:
        print(f"Error processing file: {e}")
    except FileProcessingError as e:
        print(f"General file processing error: {e}")
    except Exception as e:  # Catch any other unexpected exceptions
        print(f"Unexpected error: {e}")

process_file("data.txt")  # Might raise exceptions based on file contents and processing logic

Explanation:

• exceptions.py:
  • FileProcessingError: A base class for general file processing errors, providing a clear description using a docstring.
  • InvalidFileDataError: Inherits from FileProcessingError, specifically handling invalid data encountered in a file. Its constructor calls the base class constructor using super() for proper inheritance.
• main.py:

This enhanced example incorporates comments, a base class for file processing errors, and potential handling of additional exceptions during file processing, making the code more robust and informative.

1. Using Existing Exceptions (Context Managers):

2. Exception Groups (Contextlib):

   • If you have a set of related exceptions, consider using the contextlib module's suppress function:

   from contextlib import suppress
   
   def my_function():
       try:
           # Code that might raise multiple related exceptions
           raise ValueError("Invalid input")
       except (ValueError, TypeError):
           with suppress(ValueError, TypeError):
               handle_related_errors()  # Code to handle both errors similarly
   

   This approach suppresses specific exceptions within a block, allowing you to handle them collectively.

3. Custom Error Codes:

   • While not strictly exceptions, you can define custom error codes (e.g., integer constants) to represent specific errors. Your code can raise these codes alongside descriptive messages:

   INVALID_INPUT_ERROR = 1
   
   def my_function():
       if not is_valid(data):
           raise INVALID_INPUT_ERROR, "Input data is invalid."
   
   try:
       my_function()
   except (int, TypeError) as e:  # Catch both error code and potential type errors
       if isinstance(e, int):
           error_code = e
           error_message = "Custom error occurred: " + str(e)
       else:
           error_code = -1  # Placeholder code for unexpected errors
           error_message = str(e)
       handle_error(error_code, error_message)
   

   This method offers a lightweight approach without full-fledged exception objects. However, it requires careful handling of both the error code and message for clarity.

Remember, the best approach depends on your specific use case and the level of granularity you need in your error handling. Inheriting from Exception remains the standard and recommended method for most scenarios.