[Python-Dev] PEP 495 accepted

[Alexander Belopolsky]

On Tue, Sep 22, 2015 at 11:11 AM, Tim Peters <tim.peters at 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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