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Pure-Python ZoneInfo.fromutc() misses folds in zones with a single transition #159706
Description
Bug report
import datetime
import zoneinfo
from zoneinfo import _zoneinfo
# Requires tzdata==2026.5; use its TZif files rather than system data.
zoneinfo.reset_tzpath([])
utc = datetime.datetime(1976, 12, 1, 1, tzinfo=datetime.UTC)
for implementation in (zoneinfo, _zoneinfo):
tz = implementation.ZoneInfo.no_cache("Antarctica/Rothera")
local = utc.astimezone(tz)
print(implementation.__name__)
print(local, "fold=", local.fold)
print(local.astimezone(datetime.UTC))Output:
zoneinfo
1976-11-30 22:00:00-03:00 fold= 1
1976-12-01 01:00:00+00:00
zoneinfo._zoneinfo
1976-11-30 22:00:00+00:00 fold= 0
1976-11-30 22:00:00+00:00
The pure-Python implementation should give the same result as the C implementation. Rothera's offset moved from UTC to UTC−03 at 1976-12-01 00:00 UTC, so this instant is in the second occurrence of 22:00 local time and should have fold=1. Instead, the result has fold=0 and converting it back to UTC loses three hours.
In the single-transition branch of fromutc(), tti_prev is set to self._ttinfos[-1], which is the offset after the transition. It matches self._tz_after, so the calculated shift is zero. I think this branch needs to use self._tti_before as the previous offset, as the C implementation already does.
Linked PRs
- gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection for single-transition zones #159828
- [3.14] gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection for single-transition zones (GH-159828) #160151
- [3.15] gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection for single-transition zones (GH-159828) #160152
Activity
lazerg commented on Oct 19, 2026
I can reproduce this and have the one-line fix with a test ready. Are you planning to fix it yourself, or would you like me to open a PR?
pganssle commented on Oct 19, 2026
@lazerg Feel free to send a fix.
gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection for sing…
[3.15] gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection f…
[3.14] gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection f…