python - pytz utc conversion

Question

What is the right way to convert a naive time and a tzinfo into an UTC time? Say I have:

d = datetime(2009, 8, 31, 22, 30, 30)
tz = timezone('US/Pacific')

First way, pytz inspired:

d_tz = tz.normalize(tz.localize(d))
utc = pytz.timezone('UTC')
d_utc = d_tz.astimezone(utc)

Second way, from UTCDateTimeField

def utc_from_localtime(dt, tz):
    dt = dt.replace(tzinfo=tz)
    _dt = tz.normalize(dt)
    if dt.tzinfo != _dt.tzinfo:
        # Houston, we have a problem...
        # find out which one has a dst offset
        if _dt.tzinfo.dst(_dt):
            _dt -= _dt.tzinfo.dst(_dt)
        else:
            _dt += dt.tzinfo.dst(dt)
    return _dt.astimezone(pytz.utc)

Needless to say those two methods produce different results for quite a few timezones.

Question is - what's the right way?

Answer 1

Your first method seems to be the approved one, and should be DST-aware.

You could shorten it a tiny bit, since pytz.utc = pytz.timezone('UTC'), but you knew that already :)

tz = timezone('US/Pacific')
def toUTC(d):
    return tz.normalize(tz.localize(d)).astimezone(pytz.utc)

print "Test: ", datetime.datetime.utcnow(), " = ", toUTC(datetime.datetime.now())