Bug in time, part 2

[Joshua J. Kugler]

Or could I entitle this, "A Wrinkle in time.py," with apologies to Madeleine
L'Engle.  Either I've found a bug (or rather room for improvement) in
time.py, or I *really* need a time module that doesn't assume so much.

After the suggestions to try setting the dst flag to zero, I did some more
experimentation, and here is what I came up with:

>>> import os
>>> import time
>>> os.environ['TZ'] = 'GMT'
>>> time.tzset()
>>> time.tzname
('GMT', 'GMT')
>>> time.mktime((2007, 3, 11, 2, 30, 0, 6, 70, 0))
1173580200.0
>>> time.mktime((2007, 3, 11, 3, 30, 0, 6, 70, 0))
1173583800.0

OK, so far I'm seeing a one hour difference.

>>> time.strftime('%Y-%m-%d %H:%M:%S', time.localtime(1173580200.0))
'2007-03-11 02:30:00'
>>> time.strftime('%Y-%m-%d %H:%M:%S', time.localtime(1173583800.0))
'2007-03-11 03:30:00'

Yay! it works!  But, let's do this:

>>> import time
>>> time.tzname
('AKST', 'AKDT')
>>> int(time.mktime(time.strptime('2007-03-11 2:30:00','%Y-%m-%d %H:%M:%S'
[0:8] + (0,)))
1173612600
>>> int(time.mktime(time.strptime('2007-03-11 3:30:00','%Y-%m-%d %H:%M:%S'
[0:8] + (0,)))
1173616200
>>>

OK, that looks good, a one hour difference, and I told it not to figure in
dst (the last 0). BUT:

>>> time.strftime('%Y-%m-%d %H:%M:%S', time.localtime(1173612600)[0:8] +
(0,))
'2007-03-11 03:30:00'
>>> time.strftime('%Y-%m-%d %H:%M:%S', time.localtime(1173616200)[0:8] +
(0,))
'2007-03-11 04:30:00'

So, apparently localtime assumes DST (since we're in DST right now) and
there is no way to tell it the unixtime given to it was calculated under
the assumption of no daylight savings.  And would this particular test case
work if I told it to use DST?  Yes, but then I'd have several hundred other
test cases failing.  I think localtime needs to have a parameter to tell it
not to assume DST, even if DST would normally be active in that date range.

So, at any rate, my fix/workaround is to manually set time.py to use UTC to
convert to and from, and then deal with the times I have from there.  All I
wanted to do was convert an 18 or 19 byte character string (ISO date/time)
to a four byte character string (key in a dict, can't use ints in a shelve
db), and I end up spending HOURS trying to figure out 1) why I was getting
duplicate keys, then 2) why mktime was generating duplicate unix times, and
finally 3) why localtime wasn't converting back correctly.  Sigh.  Some
days I hate DST. :)

Thanks to all for the pointers and help.

In other news, setting time.tzname = ('GMT', 'GMT') does nothing.  You have
to set the environment variable, then call tzset.  Is there a rational for
this?  Seems odd.

j

-- 
Joshua Kugler
Lead System Admin -- Senior Programmer
http://www.eeinternet.com
PGP Key: http://pgp.mit.edu/  ID 0xDB26D7CE