[Python-ideas] Arguments to exceptions

[Ken Kundert]

On Tue, Jul 04, 2017 at 04:54:11PM -0400, Terry Reedy wrote:
> There have been many proposals for what we might call
> RichExceptions, with more easily access information.  But as Raymond
> Hettinger keeps pointing out, Python does not use exceptions only
> for (hopefully rare) errors.  It also uses them as signals for flow
> control, both as an alternative form for alternation and for
> iteration.  Alternation with try:except instead of if:else is
> common.  In the try: unicode example above, the NameError is not an
> error.  Until 2.2, IndexError served the role of StopIteration
> today, and can still be used for iteration.  For flow control,
> richer exceptions just slow code execution.

Terry,
    Correct me if I am wrong, but this seems like an argument for the proposal.
Consider the NameError, currently when raised the error message must be
constructed before it is passed to the exception. But in the proposal, you
simply pass the name (already available) and the format string (a constant). The
name is never interpolated into the format string unless the message is actually
used, which it would not in the cases you cite.

-Ken