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

Guido van Rossum guido at python.org
Tue Sep 22 16:43:23 CEST 2015

On Tue, Sep 22, 2015 at 12:01 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

It turns out there's one aspect of the accepted proposal that I think I understand, but want to confirm: the datetime -> POSIX timestamp -> datetime roundtrip for missing times.

If I'm reading the PEP correctly, the defined invariant for local times that actually exist is: dt == datetime.fromtimestamp(dt.timestamp())

Yup, except for floating point errors! Those have been fixed, finally: http://bugs.python.org/issue23517 is now closed (the fix didn't make 3.5.0; it will be in 3.5.1 though).

No confusion there for the unambiguous times, or for times in a fold. In the latter case, the timestamps produced match the points where the UTC times match the local times in the "In the Fold" UTC/local diagram.

And this is where the fold flag is essential for the roundtripping.

The subtle part is the handling of the "timestamp()" method for the "missing" times where the given time doesn't actually correspond to a valid time in the applicable timezone (local time for a naive datetime object).

Based on the UTC/local diagram from the "Mind the Gap" section, am I correct in thinking that the modified invariant that also covers times in a gap is: dt == datetime.fromtimestamp(dt.astimezone(utc).astimezone(dt.tzinfo).timestamp()) That is, for local times that exist, the invariant "dt == dt.astimezone(utc).astimezone(dt.tzinfo)" holds, but for times that don't exist, "dt.astimezone(utc).astimezone(dt.tzinfo)" will normalise them to be a time that actually exists in the original time zone, and that normalisation also effectively happens when calling "dt.timestamp()".

That can't be right -- There is no way any fromtimestamp() call can return a time in the gap. I think about the only useful invariant here is

dt.timestamp() == dt.astimezone(utc).timestamp() == dt.astimezone().timestamp()

Regards, Nick.

P.S. Thanks to whoever drew the diagrams for the In the Fold/Mind the Gap sections - I found them incredibly helpful in understanding the change!

You're welcome. It was a collaboration by myself and Alexander. I drew the first version by hand because I couldn't follow the math without a visual aid. :-)

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