[docs] [issue13277] tzinfo subclasses information

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

I would say this is a doc issue.  There are some tzinfo algorithms that depend on utcoffset(dt)-dst(dt) being invariant, but this is the part of datetime library that I have never fully understood.

What I do understand is that conversion from local time to UTC or another timezone is a hard and not always solvable problem (some local times are invalid and some are ambiguous).  (If some local government decides that 00:59 should be followed by 02:00, one is hard pressed to figure out what 01:30 local time is in UTC.)

I think documentation should emphasize the fact that the standard library only supports fixed offset timezones.  It is up to the application programmer or a 3rd party library to figure out which fixed offset is appropriate in which case.

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components: +Documentation
nosy: +tim.peters

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<http://bugs.python.org/issue13277>
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