When daylight time ends, an hour repeats on the local clock (for example,
in US Eastern, the clock jumps from 1:59 back to 1:00 again). Times in
the repeated hour are ambiguous. A tzinfo subclass that wants to play
with astimezone() needs to treat times in the repeated hour as being
standard time. astimezone() previously required that such times be
treated as daylight time. There seems no killer argument either way,
but Guido wants the standard-time version, and it does seem easier the
new way to code both American (local-time based) and European (UTC-based)
switch rules, and the astimezone() implementation is simpler.
Clarified that not all types are included. The OP was looking for a
StaticMethodType.
Also, added a note and example suggesting the use of int,str, etc.
instead of IntType, StrType, etc.
Renamed the crummy variable name in the example from "list" to "mylist".
WARNING: It would be a minor miracle if the LaTeX stuff still worked.
s/field/member/ generally everywhere, to conform with most other usage in
the docs.
s/daylight savings time/daylight saving time/ generally everywhere,
because the latter spelling is anally correct.
cases, plus even tougher tests of that. This implementation follows
the correctness proof very closely, and should also be quicker (yes,
I wrote the proof before the code, and the code proves the proof <wink>).
(or None) now. In 2.3a1 they could also return an int or long, but that
was an unhelpfully redundant leftover from an earlier version wherein
they couldn't return a timedelta. TOOWTDI.
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.
