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 but find the answer
unappealing.)
Vashek
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