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Le 03/03/2021 à 23:49, Irit Katriel via Python-Dev a écrit :
On Wed, Mar 3, 2021 at 10:39 PM Greg Ewing <greg.ewing@canterbury.ac.nz <mailto:greg.ewing@canterbury.ac.nz>> wrote:
[...] In other words, the only difference between except and except* would be that multiple except* clauses can be run, whereas only one except clause will run (the first one that matches something in the ExceptionGroup).
Is there any good reason not to do things that way?
That's an interesting idea.
Do you mean that one exception gets handled and the rest of the group is reraised? Or discarded? [...]
Hi, I'll take a shoot at this, just to see how it tastes… So, let's say: When an exception group reaches a set of traditional "except" clauses, those are examined one after the other, in the order they are in the code. That way, exceptions matched by several clauses will cause the first one to run, same as today. A subgroup is built with the subset of exceptions matched by the examined clause, as the PEP specifies for "except*". If this subgroup is None, the clause is not selected, and the next clause, if any, is examined. On the contrary, if the subgroup contains at least one matched exception, the clause is selected and no other clause will run (again, same as today). Exceptions not part of the subgroup are discarded. The clause body is then run just once (so the boss only gets one email about KeyboardInterrupt). If the clause uses the "as" form, the "as" variable is bound to one exception in the subgroup, which one is unspecified (at least for now). The other ones are discarded, except if a bare "raise" is reached (below). If a bare "raise" is reached while executing the body, the selected subgroup propagates out of the "try-except" construct. Justification: the whole group cannot propagate, because today a bare "raise" cannot reraise exceptions of a type not matched by the clause. However, if a single-type exception group is handled similar to a single exception in traditional "except" clauses, it is acceptable to let it propagate. So you would have: try: g=BaseExceptionGroup( [ValueError(), KeyboardInterrupt(), KeyboardInterrupt()]) raise g except RuntimeError: # doesn't match log_the_error() except KeyboardInterrupt as e: # builds s=g.subgroup(KeyboardInterrupt) email_the_boss(e) # tells the boss of any one error raise # reraises s except BaseException: # would match, but doesn't run launch_nuclear_attack() # BaseExceptionGroup([KeyboardInterrupt(), KeyboardInterrupt()]) # propagates further, a traditional "except KeyboardInterrupt" # would catch it. The ValueError is discarded. An interesting feature would be: when the matching clause has no "as", "except" behaves the same as "except*", apart from the fact that only one clause may run. Cheers, Baptiste