Python etc / `except*`


PEP 654 introduced not only ExceptionGroup itself but also a new syntax to handle it. Let's start right with an example:

  raise ExceptionGroup('', [
except* KeyError as e:
  print('caught1:', repr(e))
except* ValueError as e:
  print('caught2:', repr(e))
except* KeyError as e:

The output:

caught1: ExceptionGroup('', [KeyError('hello'), KeyError('world')])
caught2: ExceptionGroup('', [ValueError()])
  + Exception Group Traceback (most recent call last):
  |   File "<stdin>", line 2, in <module>
  | ExceptionGroup:  (1 sub-exception)
  +-+---------------- 1 ----------------
    | OSError

This is what happened:

  1. When ExceptionGroup is raised, it's checked against each except* block.
  2. except* KeyError block catches ExceptionGroup that contains KeyError.
  3. The matched except* block receives not the whole ExceptionGroup but its copy containing only matched sub-exceptions. In case of except* KeyError, it includes both KeyError('hello') and KeyError('world')
  4. For each sub-exception, only the first match is executed (1/0 in the example wasn't reached).
  5. While there are unmatched sub-exceptions, they will be tried to match to remaining except* blocks.
  6. If there are still sub-exceptions left after all of that, the ExceptionGroup with them is raised. So, ExceptionGroup('', [OSError()]) was raised (and beautifully formatted).