[Datetime-SIG] Another round on error-checking

[Alexander Belopolsky]

On Tue, Sep 1, 2015 at 12:58 PM, Guido van Rossum <guido at python.org> wrote:

> And given that we already have total ordering problems, from that
> perspective I could live with declaring that two datetimes that differ only
> in the fold are unequal. (Hm, aren't they already unequal because their
> utcoffset() differs?)


They are not unequal because their tzinfos are the same.  In this case
__sub__ (and as a consequence __eq__) does not call utcoffset() to follow
the rules of classic arithmetic.  My new suggestion is to use timeline
arithmetic whenever fold=1 datetime instance is involved.  This should not
break any programs that don't encounter fold=1 instances and in effect will
make fold=1 instances behave similar to how their timezone.utc equivalents
behave now.
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