[Python-Dev] PEP 348: Exception Reorganization for Python 3.0

[Raymond Hettinger]

The PEP moves StopIteration out from under Exception so that it cannot
be caught by a bare except or an explicit "except Exception".

IMO, this is a mistake.  In either form, a programmer is stating that
they want to catch and handle just about anything.  There is a
reasonable argument that SystemExit special and should float to the top,
but that is not the case with StopIteration.

When a user creates their own exception for exiting multiple levels of
loops or frames, should they inherit from ControlFlowException on the
theory that it no different in intent from StopIteration or should they
inherit from UserError on the theory that it is a custom exception?  Do
you really want routine control-flow exceptions to bypass "except
Exception".  I suspect that will lead to coding errors that are very
difficult to spot (it sure looks like it should catch a StopIteration).

Be careful with these proposals.  While well intentioned, they have
ramifications that aren't instantly apparent.  Each one needs some deep
thought, user discussion, usability testing, and a darned good reason
for changing what we already have in the field.


Raymond