In Python 3, attempting to catch in an ``except`` statement an object which does not derive from BaseException will raise a ``TypeError``. In Python 2 it is possible to raise old-style classes but this shouldn't be done anymore in order to be compatible with Python 3.
If you are about to create a custom Exception class, note that custom exceptions should inherit from ``Exception``, not ``BaseException``. ``Exception`` allows people to catch all exceptions except the ones explicitly asking the interpreter to stop, such as ``KeyboardInterrupt`` and https://docs.python.org/3/library/exceptions.html#GeneratorExit[``GeneratorExit``] which is not an error. See https://www.python.org/dev/peps/pep-0352/#exception-hierarchy-changes[PEP 352] for more information.
This rule raises an issue when the expression used in an ``except`` statement is not a class deriving from ``BaseException`` nor a tuple of such classes.
