[Datetime-SIG] Calendar vs timespan calculations...

[Alexander Belopolsky]

On Thu, Jul 30, 2015 at 8:36 PM, Tim Peters <tim.peters at gmail.com> wrote:
> Speaking of which, the current tzinfo API has no way to ask "is this
> an ambiguous time?"

I was hoping that we would agree on the name of the flag before
someone asks this question. :-)

With my proposal, a naive datetime t is ambiguous in timezone tz if

tz.utcoffset(t) < tz.utcoffset(t.replace(first=False))

> or "is this an invalid (missing) time?"

I was hoping to sneak in a rule that for an invalid time t

tz.utcoffset(t) > tz.utcoffset(t.replace(first=False))

(I really don't want  tz.utcoffset(t) to ever raise an exception) and
of course, for most of the times

tz.utcoffset(t) == tz.utcoffset(t.replace(first=False))


> The most
> important new question callers will want to resolve  is "what should
> `first` (aka is_dst) be now?".

I want most callers to be able to get away with not knowing that
`first` exists and consistently get the earlier time from an ambiguous
input and some "nearby" time from an invalid input.  A careful
application will have to call tz.utcoffset() with both values of the
flag and either warn about the default choice or ask the user for an
additional input.