[Python-Dev] PEP 495 accepted (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Wed Sep 23 07:56:32 CEST 2015
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On 23 September 2015 at 01:09, Alexander Belopolsky <alexander.belopolsky at gmail.com> wrote:
On Tue, Sep 22, 2015 at 10:55 AM, Alexander Belopolsky <alexander.belopolsky at gmail.com> wrote:
On Tue, Sep 22, 2015 at 10:43 AM, Guido van Rossum <guido at python.org> wrote:
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 don't think Nick said that. On the second reading, it looks like Nick's second sentence contradicts his first. Guido is right. Moreover, there is no way to get a time in the gap as a result of any conversion including astimezone() and fromutc() in addition to fromtimestamp(). Such datetimes may appear if you construct them explicitly, use .replace() to transplant a datetime to another timezone (or modify other components) and in the result of datetime + timedelta operation.
Sorry, what I wrote in the code wasn't what I wrote in the text, but I didn't notice until Guido pointed out the discrepancy. To get the right universal invariant, I should have normalised the LHS, not the RHS:
dt.astimezone(utc).astimezone(dt.tzinfo) ==datetime.fromtimestamp(dt.timestamp())
For unambiguous times and times in the fold, that's a subset of the stronger invariant:
dt == dt.astimezone(utc).astimezone(dt.tzinfo) ==datetime.fromtimestamp(dt.timestamp())
That stronger invariant is the one that doesn't hold for times in the gap, as with fold=0 they'll get normalised to use the right UTC offset (same UTC time, nominally an hour later local time), while with fold=1 they get mapped to an hour earlier in both UTC and local time.
Regards, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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