New submission from Paul G:
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)
----------
messages: 280007
nosy: belopolsky, p-ganssle, tim.peters
priority: normal
severity: normal
status: open
title: `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation
type: behavior
versions: Python 3.6, Python 3.7
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue28602>
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