[Python-Dev] exception chaining

Fri Jan 20 18:09:57 CET 2012

Summary:
Exception Chaining is cool, unless you are writing libraries that want 
to transform from Exception X to Exception Y as the the previous 
exception context is unnecessary, potentially confusing, and cluttery 
(yup, just made that word up!).
For all the gory details, see http://bugs.python.org/issue6210.
I'm going to attempt a patch implementing MRAB's suggestion:
try:
 some_op
except ValueError:
 raise as OtherError() # `raise` keeps context, `raise as` does not
The question I have at the moment is: should `raise as` be an error if 
no exception is currently being handled?
Example:
def smurfy(x):
 if x != 'magic flute':
 raise as WrongInstrument
 do_something_with_x
If this is allowed then `smurfy` could be called from inside an `except` 
clause or outside it.
I don't care for it for two reasons:
 - I don't like the way it looks
 - I can see it encouraging always using `raise as` instead of `raise` 
and losing the value of exception chaining.
Other thoughts?
~Ethan~