Right now there is no simple way to get a timezone-aware time for the current
time in the current time zone [without resorting to outside libraries like
pytz], I would like to propose a simple extension to datetime.now to allow
returning an aware datetime object using a plain single-offset tzinfo object:
datetime.now('auto') shall return a datetime using a simple offset-based
timezone object.
[Incidentally, I also think that utcoffset() on naive timestamps should be able
to return the offset that astimezone would use to convert the time. I only
discovered it didn't while writing this proposal]
some simplistic possible implementations:
def utcoffset_naive(dt):
ut = dt.astimezone(datetime.timezone.utc).replace(tzinfo=None)
return dt - ut
def now_auto():
now = datetime.datetime.now()
return now.replace(tzinfo=datetime.tzinfo(utcoffset_naive(now)), fold=0)
A better implementation though would be to capture tm_gmtoff, and perhaps also
tm_zone for tzname, from localtime() inside _fromtimestamp. This could also
bypass the fold detection stage because single-offset tzinfo objects have no
folds.
I am specifically *not* proposing the creation of a tzinfo class representing
"just use the local time as-is", because the meaning of timestamps with such a
tzinfo attached might change during the lifetime of a program when system
settings and/or environment variables are changed. A tzinfo class that
identifies a system-specific timezone might be viable, but on some systems may
require setting global state such as environment variables before each
conversion. Also, such a tzinfo object could simply be passed as-is to
datetime.now, whereas my proposal is to automatically select from (typically)
two different fixed-offset tzinfo objects that would be used for DST and
non-DST timestamps.
This could be accomplished with a tzinfo class whose fromutc method always
returns a datetime with a fixed-offset tzinfo attached, but I don't know if
this aspect of the tzinfo API [that fromutc may return a datetime with any
timezone attached, or indeed any object] is documented or stable, or what the
other consequences would be [e.g. what if such a tzinfo object is attached to a
datetime directly using the constructor or replace]? A magic sentinel object
such as the string 'auto' allows this functionality to be limited to now (and
perhaps fromtimestamp) without the unintended consequences of a 'magical'
tzinfo escaping into other parts of the API.
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