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.