How Should I Design Custom Exceptions in Modern Python?

The Perplexity of Custom Exception Declarations in Python

In the realm of Python exception handling, the declaration of custom exception classes has undergone significant revisions. To align with contemporary best practices, it is crucial to comprehend the current conventions.

Custom Exception Classes with Extra Data

To incorporate additional data into custom exceptions, it is now recommended to override the init method, passing any desired information as arguments. For example:

class ValidationError(Exception):
    def __init__(self, message, errors):
        # Call the base class constructor with required parameters
        super().__init__(message)
        
        # Include custom data
        self.errors = errors

This approach allows the retrieval of extra data later using e.errors.

Avoiding the Deprecation Warning

The deprecation of BaseException.message has introduced a potential pitfall. To avoid this warning, it is imperative to refrain from using the message attribute directly. Instead, override the str__, __unicode__, and __repr methods to control the representation of the exception message.

Utilizing args vs. Positional Arguments

In previous versions of Python, the use of *args was recommended for passing additional arguments to exception constructors. However, in Python 3, positional arguments should be used instead. This allows for more explicit and robust code.

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

By adhering to these principles, developers can craft custom exception classes that fully comply with modern Python standards and maintain compatibility with existing exception handling tools.

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