[Python-Dev] PEP 495 accepted (original) (raw)

Alexander Belopolsky alexander.belopolsky at gmail.com
Tue Sep 22 18:25:27 CEST 2015

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)

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.) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20150922/4d38adbe/attachment.html>

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