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
_______________________________________________
Python-ideas mailing list
Python-ideas@python.org
https://mail.python.org/mailman/listinfo/python-ideas
Code of Conduct: http://python.org/psf/codeofconduct/
