On 2016-11-29 09:43, Brett Cannon wrote:
One way to make this cheap is to have a reasonable default message and
use attributes on the exceptions trigger the use of the default message.
Nearly a year ago I filed a bunch of issues for ideas on providing
attributes on exceptions where it made sense, e.g. an index attribute on
IndexError (http://bugs.python.org/issue18162). If we did this then for
classes like IndexError there constructor could be `IndexError(index=10,
start=0, end=3)` and then __str__() can lazily construct the string
representation using a default message, e.g. `"index {} is out of range
of{} to {}".format(index, start, end)`. Make the arguments keyword-only
and they become backwards-compatible and so the only overhead you pay
for these richer messages are keyword-based construction if you simply
never access the repr for the exception.
I absolutely think this is the way to go. Having the relevant
information (the list that was too short, the index that was too big,
the key that wasn't there, etc.) is useful in many situations, and it's
much better to have that information in a programmatic form than just
squashed into an error message. This then makes it relatively easy to
write wrappers that take bubbling-up exceptions and try to construct
more detailed messages for a less experienced audience. Right now this
is difficult or impossible because the exception objects don't record
the information that would be needed for these expanded messages.
--
Brendan Barnwell
"Do not follow where the path may lead. Go, instead, where there is no
path, and leave a trail."
--author unknown
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