[Python-Dev] performance of {} versus dict()

Thu Nov 15 04:24:07 CET 2012

On Wed, Nov 14, 2012 at 8:28 PM, Stephen J. Turnbull <stephen at xemacs.org> wrote:
> Chris Angelico writes:
>> > >>> {"a":1}+{"b":2}
>> > It would make sense for this to result in {"a":1,"b":2}.
>> The test is not "does this sometimes make sense?" It's "does this
> ever result in nonsense, and if so, do we care?"
>> Here, addition is usually commutative. Should {'a':1}+{'a':2} be the
> same as, or different from, {'a':2}+{'a':1}, or should it be an error?

Easy: dict should have a (user substitutable) collision function that
is called in these cases. This would allow significant functionality
with practically no cost. In addition, it could be implemented in
such a way as to offer significant speedups (when using dict.update
for example) over any possible hand-written substitutes (since it's
only run on key collisions and otherwise uses an underlying loop coded
in C).
mark