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Author belopolsky
Recipients ajaksu2, akira, belopolsky, brett.cannon, daniel.urban, doerwalter, eric.araujo, ggenellina, kawai, l0nwlf, mark.dickinson, pitrou, r.david.murray, rafe, techtonik, tim.peters
Date 2010-06-12.16:14:00
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Message-id <1276359243.42.0.497069487211.issue5094@psf.upfronthosting.co.za>
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I have made my mind on subclassing timezone issue.  I believe subclassing should not be allowed and here is the reason:

The new datetime.timezone class is a very specific implementation of tzinfo interface.  It guarantees that utcoffset(dt) and friends return the same value for all times.  Applications should be able to rely on this and a simple isinstance(tz, timezone) should be enough to deduce that tz is a fixed offset tzinfo instance.  (Of course careful applications can check type(tz) == timezone instead, but making isinstance(tz, timezone) subtly wrong is not a good idea.)

There is also very little to be gained from subclassing timezone.  It defines only 4 methods and each is entirely trivial.  Any non-trivial tzinfo implementation will need to override all 4 of them anyways.  It is much safer to derive from tzinfo directly and possible reuse timezone through containment (for example to format offsets in a standard way.)

The only immediate application of subclassing is to add dst() method returning a fixed offset rather than None.  This is trivial to implement through containment and if such class becomes popular, I would prefer to add this functionality to timezone class itself.
History
Date User Action Args
2010-06-12 16:14:03belopolskysetrecipients: + belopolsky, tim.peters, doerwalter, brett.cannon, mark.dickinson, ggenellina, pitrou, techtonik, ajaksu2, kawai, eric.araujo, r.david.murray, rafe, daniel.urban, l0nwlf, akira
2010-06-12 16:14:03belopolskysetmessageid: <1276359243.42.0.497069487211.issue5094@psf.upfronthosting.co.za>
2010-06-12 16:14:01belopolskylinkissue5094 messages
2010-06-12 16:14:00belopolskycreate