Message280007
After PEP-495, the default value for non-fold-aware datetimes is that they return the DST side, not the STD side (as was the assumption before PEP-495). This invalidates an assumption made in `tz.fromutc()`. See lines 991-1000 of datetime.py:
dtdst = dt.dst()
if dtdst is None:
raise ValueError("fromutc() requires a non-None dst() result")
delta = dtoff - dtdst
if delta:
dt += delta
dtdst = dt.dst()
if dtdst is None:
raise ValueError("fromutc(): dt.dst gave inconsistent "
"results; cannot convert")
Line 997 (https://github.com/python/cpython/blob/be8de928e5d2f1cd4d4c9c3e6545b170f2b02f1b/Lib/datetime.py#L997) assumes that an ambiguous datetime will return the STD side, not the DST side, and as a result, if you feed it a date in UTC that should resolve to the STD side, it will actually return 1 hour (or whatever the DST offset is) AFTER the ambiguous date that should be returned.
If 997 is changed to:
dtdst = dt.replace(fold=1).dst()
That will not affect fold-naive zones (which are instructed to ignore the `fold` parameter) and restore the original behavior. This will allow fold-aware timezones to be implemented as a wrapper around `fromutc()` rather than a complete re-implementation, e.g.:
class FoldAwareTzInfo(datetime.tzinfo):
def fromutc(self, dt):
dt_wall = super(FoldAwareTzInfo, self).fromutc(dt)
is_fold = self._get_fold_status(dt, dt_wall)
return dt_wall.replace(fold=is_fold) |
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Date |
User |
Action |
Args |
2016-11-03 18:55:50 | p-ganssle | set | recipients:
+ p-ganssle, tim.peters, belopolsky |
2016-11-03 18:55:50 | p-ganssle | set | messageid: <1478199350.04.0.93782925147.issue28602@psf.upfronthosting.co.za> |
2016-11-03 18:55:50 | p-ganssle | link | issue28602 messages |
2016-11-03 18:55:49 | p-ganssle | create | |
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