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TASK OBJECTIVE

django/django

How do you add is_dst handling to Django Trunc to avoid AmbiguousTimeError and NonExistentTimeError?

pytz.exceptions.AmbiguousTimeErrorPython traceback repairs

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01

TASK STATEMENT

verbatim agent-facing issue statement

Trunc() should allow passing is_dst resolution to avoid NonExistentTimeError/AmbiguousTimeError
Description
	 
		(last modified by Alexander Holmbäck)
	 
When Trunc() truncates to a nonexisting or ambiguous datetime, the exception raised by pytz remains unhandled. The expected behavior would, IMO, be to not check the validity of truncated dates.
This test for example:
import datetime
import pytz
from django.db.models.functions import Trunc
from django.test import TestCase
from django.utils import timezone
from .models import Log
class TestTruncateToInvalidTime(TestCase):
	def test_truncate_to_dst_ends_stockholm(self):
		tzinfo = pytz.timezone('Europe/Stockholm')
		timestamp = datetime.datetime(2018, 10, 28, 2, tzinfo=tzinfo)
		Log.objects.create(timestamp=timestamp)
		logs = Log.objects.annotate(day=Trunc('timestamp', 'hour')).all()
		timezone.activate(tzinfo)
		self.assertEqual(logs[0].day.day, 28)
Results in the following error:
======================================================================
ERROR: test_truncate_to_dst_ends_stockholm (trunc.tests.TestTruncateInvalidTime)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/alex/tickets/trunc/tests.py", line 47, in test_truncate_to_dst_ends_stockholm
	self.assertEqual(logs[0].day.day, 28)
 File "/home/alex/django/django/db/models/query.py", line 303, in __getitem__
	qs._fetch_all()
 File "/home/alex/django/django/db/models/query.py", line 1190, in _fetch_all
	self._result_cache = list(self._iterable_class(self))
 File "/home/alex/django/django/db/models/query.py", line 64, in __iter__
	for row in compiler.results_iter(results):
 File "/home/alex/django/django/db/models/sql/compiler.py", line 1013, in apply_converters
	value = converter(value, expression, connection)
 File "/home/alex/django/django/db/models/functions/datetime.py", line 225, in convert_value
	value = timezone.make_aware(value, self.tzinfo)
 File "/home/alex/django/django/utils/timezone.py", line 270, in make_aware
	return timezone.localize(value, is_dst=is_dst)
 File "/home/alex/.virtualenvs/djangodev/lib/python3.6/site-packages/pytz/tzinfo.py", line 363, in localize
	raise AmbiguousTimeError(dt)
pytz.exceptions.AmbiguousTimeError: 2018-10-28 02:00:00

Match evidence

Primary terms

pytz.exceptions.AmbiguousTimeErrorTruncdjango/db/models/functions/datetime.pydjango/djangoTruncBaseconvert_valueis_dstmake_awaretimezone.localizeapply_convertersfrom django.db.models.functions import Truncfrom django.utils import timezone

Technical objective

Give callers of Django's database date/time truncation functions a way to control how an ambiguous or non-existent local datetime is resolved when the truncated value is converted back to an aware datetime. The deliverable is a change to the Trunc-family database functions (and whatever timezone helper they rely on) so that truncation to a DST transition boundary no longer surfaces an unhandled pytz exception to the ORM caller.

Success condition

a queryset annotation that truncates a timestamp onto a non-existent or ambiguous local time can be evaluated without raising, with the resolution behaviour selectable by the caller; existing Trunc results for unambiguous datetimes and the current default behaviour of the timezone helpers stay intact.

Search fingerprints

  • Error signature: pytz.exceptions.AmbiguousTimeError
  • Traceback path: django/db/models/functions/datetime.py
  • Repository: django/django
  • Issue title: Trunc() should allow passing is_dst resolution to avoid NonExistentTimeError/AmbiguousTimeError.
  • Symbols: Trunc, TruncBase, convert_value, is_dst, make_aware, timezone.localize, apply_converters.
  • Imports: from django.db.models.functions import Trunc, from django.utils import timezone.
  • Files: django/db/models/functions/datetime.py, django/utils/timezone.py, django/db/models/sql/compiler.py, django/db/models/query.py.
  • Exceptions: pytz.exceptions.AmbiguousTimeError, NonExistentTimeError.
  • Verbatim strings: pytz.exceptions.AmbiguousTimeError: 2018-10-28 02:00:00, ERROR: test_truncate_to_dst_ends_stockholm (trunc.tests.TestTruncateInvalidTime).
  • Technologies: Django ORM annotations, pytz, DST transitions, Europe/Stockholm.

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