try:
    <smth>
except (MultiError, ValueError) as e:
    def _handle(e):
        if isinstance(e, ValueError):
            return None
        else:
            return exc
    MultiError.filter(_handle,e)

So a statement that the current handling is "cumbersome" and "unintuitive" is unconvincing.

If this results in lots of boilerplate code with isinstance(), filter() can be changed to e.g. accept a dict of exception types and handlers. Actually, I wonder why you didn't do that already if handling specific exception types and reraising the rest is the standard procedure!
Then the code would be reduced to:

try:
    <smth>
except (MultiError, ValueError) as e:
    MultiError.filter({ValueError: lambda _: None},
                      e)


If some code doesn't know about MultiError's, it should handle one like any other unknown exception that it cannot do anything intelligent about.
If it wishes to handle them, you need to split MultiError into a separate library that anyone could use without having to pull the entire `trio`.


On 23.02.2021 3:24, Irit Katriel via Python-Dev wrote:

Hi all,

We would like to request feedback on PEP 654 -- Exception Groups and except*.

https://www.python.org/dev/peps/pep-0654/

It proposes language extensions that allow programs to raise and handle multiple unrelated
exceptions simultaneously, motivated by the needs of asyncio and other concurrency libraries,
but with other use cases as well.

* A new standard exception type,  ExceptionGroup, to represent multiple exceptions with
  shared traceback.
* Updates to the traceback printing code to display (possibly nested) ExceptionGroups.
* A new syntax except* for handling ExceptionGroups.

A reference implementation (unreviewed) can be found at:
https://github.com/iritkatriel/cpython/pull/10

Thank you for your help

Kind regards
Irit, Yury & Guido



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-- 
Regards,
Ivan