[Python-Dev] Adding a tzidx cache to datetime

[Victor Stinner]

Le ven. 10 mai 2019 à 09:22, M.-A. Lemburg <mal at egenix.com> a écrit :
> Given that many datetime objects in practice don't use timezones
> (e.g. in large data stores you typically use UTC and naive datetime
> objects), I think that making the object itself larger to accommodate
> for a cache, which will only be used a smaller percentage of the use
> cases, isn't warranted. Going from 64 bytes to 72 bytes also sounds
> like this could have negative effects on cache lines.
>
> If you need a per object cache, you can either use weakref
> objects or maintain a separate dictionary in dateutil or other
> timezone helpers which indexes objects by id(obj).
>
> That said, if you only add a byte field which doesn't make the object
> larger in practice (you merely use space that alignments would
> use anyway), this shouldn't be a problem. The use of that field
> should be documented, though, so that other implementations can
> use/provide it as well.

>From Marc-Andre Lemburg, I understand that Paul's PR is a good
compromise and that other datetime implementations which cannot use
tzidx() cache (because it's limited to an integer in [0; 254]) can
subclass datetime or use a cache outside datetime.

Note: right now, creating a weakref to a datetime fails.

Victor
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