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From: Tim Peters <tim.peters@gmail.com>
Date: Mon, 14 Sep 2015 14:49:53 -0500
Subject: Re: [Datetime-SIG] Are there any "correct" implementations of tzinfo?
To: Alexander Belopolsky <alexander.belopolsky@gmail.com>
Cc: Random832 <random832@fastmail.com>, Python-List <python-list@python.org>, datetime-sig <datetime-sig@python.org>
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[Tim]
>> It would be nice to have!  .utcoffset() is an expensive operation
>> as-is, and being able to rely on tm_gmtoff would make that dirt-cheap
>> instead.

[Alex]
> If it  is just a question of optimization,

Yes.  If it's more than just that, then 495 doesn't actually solve the
problem of getting the correct UTC offset in all cases.


> datetime objects can be extended to cache utcoffset.  Note that PyPy
> have recently added caching of the hash values in datetime objects.  I
> merged their changes in our datetime.py, but it did not look like C
> implementation would benefit from it as much as pure python did.  I
> expect something similar from caching utcoffset: a measurable
> improvement for tzinfos implemented in Python and a wash for those
> implemented in C.  (A more promising optimization approach is to define a C
> API for tzinfo interface.)

There's no answer to this.  It depends on how expensive .utcoffset()
is, which in turn depends on how the tzinfo author implements it.

I don't care now fast it is.  But, even if I did, "premature
optimization" applies at this time ;-)