On 3/4/2021 1:41 AM, Irit Katriel wrote:

On Thu, Mar 4, 2021 at 1:38 AM Glenn Linderman <v+python@g.nevcal.com> wrote:
On 3/3/2021 2:49 PM, Irit Katriel via Python-Dev wrote:
That's an interesting idea. 

Do you mean that one exception gets handled and the rest of the group is reraised? Or discarded?

The value of sys.exc_info() (and the e in "except T as e:") needs to be a single naked exception. So if there is more than one match in the group we would need to pick one (let's say the first in DFS order).

If we do this, then we have this situation. Before ExceptionGroups, you got to choose which of the exceptions you have is the most important, and you raised only that one. Now you raise a bunch of them and the order of the except clauses in caller's code determines which one of them counts and which ones are discarded. What do you make of that?

You _could_ implement it as you said, but remember, you that with this idea, you are changing how except clauses work—so instead of making the order of the except clauses determine which one counts most, you could instead do something else.

One alternative idea would be to take the "first in DFS order" and see if it matches any of the except clauses, and if so, process that one.  If not, then pick the next, and see if it matches, until one is found that matches, and can be processed.

Or we could make it explicit: 

add an optional arg to ExceptionGroup like
ExceptionGroup("eg", list_of_exceptions, singleton=None) 

In the example of atexit, where currently it raises only the last exception from your callbacks, it will instead raise

ExceptionGroup("atexit errors", all_exceptions, singleton=last_exception)

Then except* works as before, ignoring the singleton.  But except matches the singleton.

And there's no magic where you can be surprised about which exception except chose to look at.

I like explicit, and avoiding magic.

And this gives a compatibility story for outer loops that except: Exception, and even for others cases that are not recoded for ExceptionGroup handling.

And I guess what you are citing is a precedent from atexit, for raising the last one.

And I guess in cases other than atexit, when raising an ExceptionGroup, the coder of the new feature would still get more of a choice about which Exception is more important, rather than the coder of the except clauses.  One could quibble that if ValueError and IndexError were both part of the ExceptionGroup, that if the except clauses expected either might happen, and listed them in ValueError and IndexError order, that the intention might have been that ValueError was more interesting to the except clause coder, whereas if the group is raised with the IndexError as the singleton, that the group coder has the opposite intention. I think it is more likely that the except clause coder simply knew they were mutually exclusive and that the order of the clauses didn't matter.

Thinking about the above a bit, the only existing except clause sequence that would matter would be if both a base exception class and a derived class were both listed in the except clauses.  The derived exception class should be listed before the base exception class or it wouldn't get processed. So it is not clear that the order of the except clauses really indicates any priority of interest on the part of the except clause coder?