[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Alexander Belopolsky added the comment: Paul, In your opening post to this issue you suggested to change one line [1] in Lib/datetime.py from dtdst = dt.dst() to dtdst = dt.replace(fold=1).dst() This looks like a rather innocuous change, but it does not by itself make fromutc() return pr

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Paul Ganssle added the comment: > I am aware of that. In fact, some of the draft versions of PEP 495 > implementation did contain such code. The problem is that any such > tz.fromutc() implementation would necessarily change the behavior of the old > programs. This I'm not sure about - th

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Alexander Belopolsky added the comment: > it's actually possible to implement full `fold` support in a generic way I am aware of that. In fact, some of the draft versions of PEP 495 implementation did contain such code. The problem is that any such tz.fromutc() implementation would necess

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Paul Ganssle added the comment: So I created a little patch for this, and when I was working on it I realized that it's actually possible to implement full `fold` support in a generic way in `fromutc` under the assumption that `utcoffset - dst` holds at all points (which is the fundamental a

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Change by Paul Ganssle : -- keywords: +patch pull_requests: +7050 stage: needs patch -> patch review ___ Python tracker ___ ___ Pyth

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Change by Alexander Belopolsky : -- stage: -> needs patch versions: -Python 3.6, Python 3.7 ___ Python tracker ___ ___ Python-bugs

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Change by Paul Ganssle : -- versions: +Python 3.8 ___ Python tracker ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Alexander Belopolsky added the comment: > I can't think of a single actual downside to this change - all it does is > preserve the original behavior of `fromutc()`. You've got me on the fence here. If what you are saying is correct, it would make sense to make this change and better do it bef

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Paul G added the comment: > After all, how much effort would it save for you in dateutil if you could > reuse the base class fromutc? Realistically, this saves me nothing since I have to re-implement it anyway in in all versions <= Python 3.6 (basically just the exact same algorithm with line

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Alexander Belopolsky added the comment: Paul G at Github: """ To be clear, I'm just saying that fromutc() goes from returning something meaningful (the correct date and time, with no indication of what side of the fold it's on) to something useless (an incorrect date and time) in the case whe

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Paul G added the comment: Of the `tzinfo` implementations provided by `python-dateutil`, `tzrange`, `tzstr` (GNU TZ strings), `tzwin` (Windows style time zones) and `tzlocal` all satisfy this condition. These are basically all implementations of default system time zone information. With curr

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

Alexander Belopolsky added the comment: I don't think timezones that satisfy the invariant expected by the default fromutc() is common enough that we need to provide special support for them. Note that in most cases it is the UTC to local conversion that is straightforward while Local to UTC

[issue28602] `tzinfo.fromutc()` fails when used for a fold-aware tzinfo implementation

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()