[Python-Dev] Status on PEP-431 Timezones

[Isaac Schwabacher]

On 15-04-08, Lennart Regebro wrote:
> === Stdlib option 2: A datetime _is_dst flag ===
> 
> By having a flag on the datetime instance that says "this is in DST or not"
> the timezone implementation can be kept simpler.

Storing the offset itself instead of a flag makes things conceptually cleaner. You get a representation that's slightly harder to construct from the sorts of information you have lying around (tz name, naive datetime, is_dst flag) but is no harder to construct *and* validate, and then is easier to work with and harder to misuse. As an added bonus, you get a representation that's still meaningful when time changes happen for political rather than DST reasons.

Pros:
- tzinfo.utcoffset() and local->UTC conversions don't require zoneinfo access.
- it's harder to represent "I know this time is DST but I don't know what tz it's in" [I put this in pro because I don't see how this kind of ambiguity can lead to anything but trouble, but ymmv]
- this representation is meaningful regardless of whether a time zone has DST
- this representation meaningfully disambiguates across time changes not resulting from DST
Cons:
- tzinfo.dst() requires zoneinfo access
- tzinfo.tzname() requires zoneinfo access IF you want those horrible ambiguous abbreviations (is CST America/Chicago or America/Havana?) [I really wanted to put this in pro]
- (datetime, offset, tz) triples require validation [but so do (datetime, is_dst, tz) triples, even though it's easy to pretend they don't]
- constructing an aware datetime from a naive one, an is_dst flag, and a time zone requires zoneinfo access [but this is needed for the validation step anyway]

On 15-04-08, Alexander Belopolsky wrote:
> With datetime, we also have a problem that POSIX APIs don't have to deal with: local time
> arithmetics. What is t + timedelta(1) when t falls on the day before DST change? How would
> you set the isdst flag in the result?

It's whatever time comes 60*60*24 seconds after t in the same time zone, because the timedelta class isn't expressive enough to represent anything but absolute time differences (nor should it be, IMO). But it might make sense to import dateutil.relativedelta or mxDateTime.RelativeDateTime into the stdlib to make relative time differences (same local time on the next day, for instance) possible.

ijs