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.
understood now: it can't work. Added comments explaining why (it's "the
usual"-- unrepresentable hours in local time --but in a slightly different
guise).
an idea from Guido. This restores that the datetime implementation
never passes a datetime d to a tzinfo method unless d.tzinfo is the
tzinfo instance whose method is being called. That in turn allows
enormous simplifications in user-written tzinfo classes (see the Python
sandbox US.py and EU.py for fully fleshed-out examples).
d.astimezone(tz) also raises ValueError now if d lands in the one hour
of the year that can't be expressed in tz (this can happen iff tz models
both standard and daylight time). That it used to return a nonsense
result always ate at me, and it turned out that it seemed impossible to
force a consistent nonsense result under the new implementation (which
doesn't know anything about how tzinfo classes implement their methods --
it can only infer properties indirectly). Guido doesn't like this --
expect it to change.
New tests of conversion between adjacent DST-aware timezones don't pass
yet, and are commented out.
Running the datetime tests in a loop under a debug build leaks 9
references per test run, but I don't believe the datetime code is the
cause (it didn't leak the last time I changed the C code, and the leak
is the same if I disable all the tests that invoke the only function
that changed here). I'll pursue that next.
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.
Guido has in mind an easier way for users to code this stuff, but the
only tests we have now are for fixed-offset tzinfo classes, and this
stuff is extremely delicate in the endcases (read the new test code
for why: there are holes in time <wink>).
operands have identical tzinfo members (meaning object identity -- "is").
I misunderstood the intent here, reading wrong conclusion into
conflicting clues.
subtraction, work as documented. In the Python implementation,
they weren't calling utcoffset() if both operands had the same
tzinfo object. That's fine if it so happens that the shared
tzinfo object returns a fixed offset (independent of operand),
but can give wrong results if that's not so, and the latter
obtains in a tzinfo subclass instance trying to model both
standard and daylight times. The C implementation was already
doing this "correctly", so we're just adding tests to verify it.
be trusted with years before 1900, so now we raise ValueError if a date or
datetime or datetimetz .strftime() method is called with a year before
1900.
{timetz,datetimetz}.{utcoffset,dst}() now return a timedelta (or None)
instead of an int (or None).
tzinfo.{utcoffset,dst)() can now return a timedelta (or an int, or None).
Curiously, this was much easier to do in the C implementation than in the
Python implementation (which lives in the Zope3 code tree) -- the C code
already had lots of hair to extract C ints from offset objects, and used
C ints internally.
used that.
wrap_strftime(): Removed the most irritating uses of buf.
TestDate.test_ordinal_conversions(): The C implementation is fast enough
that we can afford to check the endpoints of every year. Also added
tm_yday tests at the endpoints.
Tim Peters