[Python-Dev] shrinking dicts

[Tim Peters]

[Tim]
>> ...slowing the expected case to prevent theoretical bad cases is
>> usually a net loss -- I think the onus is on you to demonstrate
>> that this change is an exception to that rule.

[Vladimir Marangozov]
> I won't, because this case is rare in practice, classifying it already
> as an exception. A real exception. I'll have to think a bit more about
> all this. Adding 1/3 new entries to trigger the next resize sounds
> suboptimal (if it happens at all).

"Suboptimal" with respect to which specific cost model?  Exhibiting a
specific bad case isn't compelling, and especially not when it's considered
to be "a real exception".  Adding new expense to every delete is an obvious
new burden -- where's the payback, and is the expected net effect amortized
across all dict usage a win or loss?  Offhand it sounds like a small loss to
me, although I haven't worked up a formal cost model either <wink>.

> ...
> I think that officializing these tricks in the documentation is a
> bad idea.

It's rarely a good idea to keep truths secret, although
implementation-du-jour tricks don't belong in the current doc set.  Probably
in a HowTo.

>> if-it-wasn't-a-problem-the-first-8-years-of-python's-life-it's-hard-
>>     to-see-why-1999-is-special-ly y'rs  - tim

> This is a good (your favorite ;-) argument,

I actually hate that kind of argument -- it's one of *Guido's* favorites,
and in his current silent state I'm simply channeling him <wink>.

> but don't forget that you've been around, teaching people various
> tricks.

As I said, this particular trick has come up only once in real life in my
experience; it's never come up in my own code; it's an anti-FAQ.  People are
100x more likely to whine about theoretical quadratic-time list growth
nobody has ever encountered (although it looks like they may finally get it
under an out-of-the-box BDW collector!).

> And 1999 is special -- we just had a solar eclipse today, the next being
> scheduled for 2081.

Ya, like any of us will survive Y2K to see it <wink>.

1999-is-special-cuz-it's-the-end-of-civilization-ly y'rs  - tim