How Can I Effectively Raise and Handle Exceptions in Python?

Manually Raising a Python Exception

In Python, exceptions can be manually triggered to be handled in an except block. To do so, use the specific Exception constructor that best aligns with your situation.

Benefits and Drawbacks

Throwing specific exceptions offers several advantages:

• Precision: It allows for targeted error handling, ensuring that relevant exceptions are handled appropriately.
• Debugging: Accurate error messages assist in identifying and resolving issues effectively.

Conversely, using generic exceptions can obscure bugs and prevent specific error handling for subclasses.

Best Practices

raise Statement:

• Use specific Exception classes aligned with the issue.
• Include an informative message describing the error.
• Leverage the args attribute for detailed error information.

raise ValueError('A very specific error occurred!')

except Clause:

• Log exception details if needed.
• Rethrow exceptions using a bare raise statement to preserve the stack trace.

try:
    ...
except AppError as error:
    logger.error(error)
    raise

Modifying Errors:

While preserving the stacktrace with sys.exc_info() is possible, it's prone to errors and compatibility issues. Use the bare raise mechanism to re-raise exceptions instead.

Exception Chaining (Python 3):

This feature allows exceptions to be chained, preserving tracebacks.

raise RuntimeError('specific message') from error

Deprecated Methods:

Avoid using deprecated methods like raise ValueError, 'message' or raising strings directly, which can result in hidden errors.

Custom Exception Types:

When required, create custom exception types to indicate specific application errors.

class MyAppException(Exception):
    '''Custom error for application-related issues'''

Example:

def api_function(foo):
    if foo not in _ALLOWED_ARGS:
        raise ValueError(f'{foo} is invalid. Use "baz" or "bar" instead.')

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