#29754: Trunc() should allow passing is_dst resolution to avoid
NonExistentTimeError/AmbiguousTimeError
-------------------------------------+-------------------------------------
Reporter: Alexander Holmbäck | Owner: Alexander
| Holmbäck
Type: New feature | Status: assigned
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: pytz, Trunc, is_dst | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
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Changes (by Alexander Holmbäck):
* needs_better_patch: 0 => 1
Comment:
Ok I'v been digging a bit deeper into this, here's a recap and a few
proposals.
When `make_aware()` is fed a datetime that isn't valid in the current
timezone, it convert it to standard time if `is_dst=False` and daylight
saving time if `is_dst=True`. If `is_dst=None`, which is default, it
raises an exception. All this seems reasonable, it departs from `pytz`
default which is to convert the invalid datetime to normal time, but
that's justifieable and well
[https://docs.djangoproject.com/en/2.1/ref/utils/#django.utils.timezone.make_aware
documented].
In the case of `Trunc()` however, having `make_aware()` raise an excpetion
when a truncated date is invalid is overly drastic. Hence, the current
patch introduces a new optional flag `is_dst` to `TruncBase()` so that
developers can decide for themselves how invalid times should be treated.
But the consequences of not setting that flag can be dire and predicting
those consequences would be difficult for all but the must timezone savvy
developers (I know I wouldn't).
That said, I've been thinking about alternative solutions.
1) Pass `is_dst=False` to `make_aware()`. This would resonate with the
behavior of `pytz` but it covers up invalid datetimes and a developer
would need to re-localize them to know which datetime is valid and which
isn't. I's also odd to explicitly choose standard time for an invalid
datetime, even if that's what `pytz` does.
2) Keep invalid datetimes naive. This would make invalid datetimes easier
to distinguish from valid datetimes but as they are naive they wouldn't
always behave like aware datetimes which could cause other problems down
the line.
3) Create a subclass from `pytz.tzinfo.DstTzInfo` called
`InvalidDstTzInfo` and attach that to `tzinfo` for invalid dates. This
would make invalid datetimes behave just like valid datetimes but also
easy to recognize. This could be neat but it entangles django's `timezone`
functionality with `pytz` in ways that will make it more difficult to
maintain.
I'm leaning slightly towards the first alternative. If we want to help
developers checking for invalid dates we could do as `dateutil` does and
provide `timezone.datetime_ambiguous(dt)` and
`timezone.datetime_exists(dt)` as helper functions.
Thoughts?
--
Ticket URL: <https://code.djangoproject.com/ticket/29754#comment:11>
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