[Python-ideas] suppressing exception context when it is not relevant

[MRAB]

On 2016-10-11 01:43, Václav Dvořák wrote:
> I'm aware of "raise ... from None" (from PEP 415). However, how can I
> achieve that same effect (of suppressing the "During handling of the
> above exception, another exception occurred" message) without having
> control over the code that is executed from the except clause? I thought
> that sys.exc_clear() could be used for this, but that function doesn't
> exist in Python 3 anymore.
>
> Why would I want this? I have some simple caching code that looks like
> (simplified):
>
>     try:
>         value = cache_dict[key]
>     except KeyError:
>         value = some_api.get_the_value_via_web_service_call(key)
>         cache_dict[key] = value
>
>
> When there's an exception in the API call, the output will be something
> like this:
>
>     Traceback (most recent call last):
>       File ..., line ..., in ...
>     KeyError: '...'
>
>     During handling of the above exception, another exception occurred:
>
>     Traceback (most recent call last):
>       File ..., line ..., in ...
>     some_api.TheInterestingException: ...
>
>
> But I find this misleading, as the original KeyError is not really an
> error at all. I could of course avoid the situation by changing the
> try/except (EAFP) into a test for the key's presence (LBYL) but that's
> not very Pythonic and less thread-friendly (not that the above is
> thread-safe as is, but that's beside the point). Also, yes, I could
> instead subclass dict and implement __missing__, but that's only a
> solution for this particular case. The problem (if you agree it's a
> problem) occurs any time an exception is not actually an error, but
> rather a condition that just happens to be indicated by an exception.
>
> It's unreasonable to expect all code in some_api to change their raise
> X to raise X from None (and it wouldn't even make sense in all cases).
> Is there a clean solution to avoid the unwanted exception chain in the
> error message?
>
> If not, would it make sense to re-introduce sys.exc_clear() for this
> purpose?
>
> (I originally asked about this
> here: http://stackoverflow.com/questions/30235516/how-to-suppress-displaying-the-parent-exception-the-cause-for-subsequent-excep
> <http://stackoverflow.com/questions/30235516/how-to-suppress-displaying-the-parent-exception-the-cause-for-subsequent-excep> but
> find the answer unappealing.)
>
You could use a sentinel instead:

     MISSING = object()
     value = cache_dict.get(key, MISSING)
     if value is MISSING:
         value = some_api.get_the_value_via_web_service_call(key)
         cache_dict[key] = value