On Tue, Sep 22, 2015 at 11:11 AM, Tim Peters <tim.pet...@gmail.com> wrote:
> [Nick Coghlan] > ... > >>> dt == > >>> > datetime.fromtimestamp(dt.astimezone(utc).astimezone(dt.tzinfo).timestamp()) > ... > [Guido] > >> That can't be right -- There is no way any fromtimestamp() call can > return > >> a time in the gap. > > [Alexander Belopolsky] > > I don't think Nick said that. > > [Tim Peters] > I do, except that he didn't ;-) Count the parens carefully. > OK, it looks like Nick has managed to confuse both authors of the PEP, but not Guido. :-) The .astimezone() conversions in Nick's expression are a red herring. They don't change the value of the timestamp. That's the invariant Guido mentioned: dt.timestamp() == dt.astimezone(utc).timestamp() == dt.astimezone(utc).astimezone(dt.tzinfo).timestamp() Now, if dt is in its tzinfo gap, then dt != datetime.fromtimestamp(dt.timestamp(), dt.tzinfo) Instead, you get something like this: datetime.fromtimestamp(dt.timestamp(), dt.tzinfo) == dt + (1 - 2*dt.fold) * gap where gap is the size of the gap expressed as a timedelta (typically gap = timedelta(hours=1)). In words, when you ask for 2:40 AM, but the clock jumps from 01:59 AM to 03:00 AM, the round trip through timestamp gives you 03:40 AM if fold=0 and 01:40 AM if fold=1. This rule is somewhat arbitrary, but it has many nice mathematical and "human" properties. (There is an (imperfect) analogy with the roots of a quadratic equation here: when no real solutions exist, the two complex solutions are a ± i*b and "nice to have" real values are a ± b.)
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