Django Design Patterns and Best Practices - Second Edition

Exception syntax changes and improvements

Exception handling syntax and functionality has been significantly improved in Python 3.

In Python 3, you cannot use the comma-separated syntax for the except clause. Use the as keyword instead:

Python 2	Python 3 and 2
try: pass except e, BaseException: pass	try: passexcept e as BaseException: pass

The new syntax is recommended for Python 2 as well.

In Python 3, all exceptions must be derived (directly or indirectly) from BaseException. In practice, you will create your custom exceptions by deriving from the Exception class.

As a major improvement in error reporting, if an exception occurs while handling an exception, the entire chain of exceptions is reported:

Python 2

Python ...