[Datetime-SIG] PEP-495 - Strict Invalid Time Checking

[Alexander Belopolsky]

On Tue, Aug 25, 2015 at 3:36 PM, Carl Meyer <carl at oddbird.net> wrote:

> Given the unlikelihood of someone trying to schedule an appointment at
> 2am, I didn't care much about the specific choice of user experience for
> the DST gaps and folds; my overriding concerns were to a) not crash the
> application when displaying a DST-transition day, and b) minimize the
> harm done to my codebase (in fragility or added complexity) for the sake
> of handling this rare edge case.
>

Here is my recommendation for your case.  Assuming that taking user entry
is equivalent to parsing date and time string in some format and scheduling
is equivalent to writing a seconds since epoch timestamp into some
database, do the following post-PEP 495:

dt = datetime.strptime(input, format)
s = dt.timestamp()
schedule(s)
# optionally
sdt = datetime.fromtimestamp(s)
if dt != sdt:
    warn("You specified an invalid time %s, we scheduled your appointment
for %s", dt, sdt)

Instead of a warning, your program may highlight the changed hour in read.

This program will have no means of scheduling in the "fold=1" hour and will
correct you if you enter time that falls in a gap.  The PEP 495 design is
motivated by precisely this type of application.
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