RuntimeError, or user defined exception

[Cameron Simpson]

On 04Feb2022 21:39, Cecil Westerhof <Cecil at decebal.nl> wrote:
>I am creating a class that will call a user defined function on user
>defined intervals. In my opinion it is an error when the function
>takes more as 10% of interval, or more as half a second. What is
>better: just using RuntimeError, or creating two exception classes for
>this?

You could reuse TimeoutError, a builtin exception raised by OS calls 
which time out: https://docs.python.org/3/library/exceptions.html#TimeoutError

I wouldn't use RuntimeError. Its core purpose is interpreter failures 
IIRC, and I use it myself for situations which should never occur - 
things indicating an actual bug in my code.

Specificly, I do not expect to try to catch RuntimeError, and broadly 
don't expect anything else too either (with some narrow exceptions).  
Whereas you would very reasonably catch TimeoutError as a situation 
which can readily occur. _Particularly_ with a tool which runs code 
supplied from outside - it _might_ do anything.

IMO your situation is a misbehaviour, not an internal bug. I'd reach for 
TimeoutError.

An example from my own code where I use a RuntimeError:

    class FSTagsCommand(BaseCommand, TagsCommandMixin):
      ''' `fstags` main command line utility.
      '''
      GETOPT_SPEC = 'o:'
      [.......]
      def apply_opt(self, opt, val):
        ''' Apply command line option.
        '''
        options = self.options
        if opt == '-o':
          options.ontology_path = val
        else:
          raise RuntimeError("unhandled option")

Here, the BaseCommand superclass parses command line options according 
to GETOPT_SPEC. The apply_opt() method gets called with any valid 
options. GETOPT_SPEC='o:', so seeing anything except '-o' in apply_opt() 
indicates a mismatch between GETOPT_SPEC and my apply_opt implementation 
(other options get rejected by BaseCommand). So _if_ my code gets there, 
that is an internal bug of my own.

Cheers,
Cameron Simpson <cs at cskk.id.au>