[Datetime-SIG] Are there any "correct" implementations of tzinfo?

Random832 random832 at fastmail.com
Mon Sep 14 23:23:20 CEST 2015

On Mon, Sep 14, 2015, at 16:45, Tim Peters wrote:
> Because all versions of Python expect a very specific pickle layout
> for _every_ kind of pickled object (including datetimes)..  Make any
> change to the pickle format of any object, and older Pythons will
> simply blow up (raise an exception) when trying to load the new pickle
> - or do something insane with the pickle bits.  It's impossible for
> older Pythons to know anything about what "the new bits" are supposed
> to mean, and there is no way to spell, in the pickle engine, "but if
> you're an older version, skip over the next N bytes".

Well, you could have put some reserved bits in the original pickle
format for datetime back when it was first defined, or even just allowed
passing in a longer string for future extension purposes. That you
didn't makes me wonder just where you're finding the space to put the
fold bit.

> It's not so much a "good idea" as that it's the only idea consistent
> with Python's "naive time" model.  Folds and gaps don't exist in naive
> time.  Indeed, the _concept_ of "time zone" doesn't really exist in
> naive time.  There's _inherent_ tension between the naive time model
> and the way multi-offset time zones actually behave.  So it goes.

But why does it need to be consistent? You can't compare naive datetimes
with aware ones. If you want to sort/bisect a list of datetimes, they
have to either all be naive or all be aware. So when we're talking about
how ordering works, we're fundamentally talking about how it works for
aware datetimes.

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