On Tue, 29 Nov 2016 at 02:39 Nathaniel Smith <n...@pobox.com> wrote: > On Tue, Nov 29, 2016 at 1:05 AM, Victor Stinner > <victor.stin...@gmail.com> wrote: > > Hi, > > > > Python is optimized for performance. Formatting an error message has a > > cost on performances. > > Sure, but we have to look at this on a case-by-case basis. Is there > really important code out there that's generating NameErrors or > SyntaxErrors in an inner loop? That seems unlikely to me. > > Even IndexError I'm a bit skeptical about. I can believe that there's > code that intentionally generates and then catches IndexError, but > AttributeError in my experience is much more performance-sensitive > than IndexError, because every failed hasattr call allocates an > AttributeError and hasattr is commonly used for feature checks. Yet > AttributeError has a much more informative (= expensive) message than > IndexError: > > In [1]: object().a > AttributeError: 'object' object has no attribute 'a' > > In [2]: list()[0] > IndexError: list index out of range >
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.
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