suggestion from Guido, along with a formal correctness proof of the
trickiest bit. The intricacy of the proof reveals how delicate this
is, but also how robust the conclusion: correctness doesn't rely on
dst() returning +- one hour (not all real time zones do!), it only
relies on:
1. That dst() returns a (any) non-zero value if and only if daylight
time is in effect.
and
2. That the tzinfo subclass implements a consistent notion of time zone.
The meaning of "consistent" was a hidden assumption, which is now an
explicit requirement in the docs. Alas, it's an unverifiable (by the
datetime implementation) requirement, but so it goes.
docs. Replaced it with an XXX block, because the hoped-for treatment
of DST endcases remains unclear (Guido doesn't really like raising an
exception when it's impossible to deliver a correct result, but so
far I have no way in hand to consistently deliver a defined incorrect
result either).
of the timetz case. A tzinfo method will always see a datetimetz arg,
or None, now. In the former case, it's still possible that it will get
a datetimetz argument belonging to a different timezone. That will get
fixed next.
operands have identical tzinfo members (meaning object identity -- "is").
I misunderstood the intent here, reading wrong conclusion into
conflicting clues.
Tim Peters
Fred Drake
Raymond Hettinger
Guido van Rossum
Andrew M. Kuchling