[Zope3-dev] timetuple and utctimetuple

[Marius Gedminas]

Hi,

Today I was playing with the datetime module again, and I noticed a bug:

  date(2001, 1, 3).strftime('%Y-%m-%d') == "2001-01-02"

if you happen to live west from UTC.  The bug happens because strftime
passes a time_t timestamp (which is always in UTC) to localtime, and
localtime adds your timezone offset before conversion to a tuple.  The
fix is to use gmtime, which returns the same date you manually
constructed.  I've just commited this fix to datetime in Zope3 CVS.

Grepping around for other uses of time.localtime I noticed one in
date.timetuple().  Assuming that this is the same bug, and that you
always want the following equality to hold

  date(y, m, d).timetuple()[:3] == (y, m, d)

I applied and commited the same fix (only I accidantally pasted the unit
test to TestDateTime instead of TestDate, will fix that later).

Then I noticed utctimetuple() and began to have doubts.  Maybe I
shouldn't have "fixed" timetuple()?  What should be the exact semantics
of those methods?  Assume the following code:

  tt1 = dt.timetuple()
  tt2 = dt.utctimetuple()

Without my "fix" the following holds:

  tt1 is a time tuple in local time when dt is in UTC
  tt1 is not meaningful when dt is in local time

  tt2 is a time tuple in UTC when dt is in UTC
  tt2 is a time tuple in local time when dt is in localtime

Personally, I find it a bit confusing.  I think that dt.timetuple()
should return a broken-down time representation in the same timezone dt
was defined in.

With my "fix"

  tt1 is a time tuple in local time when dt is in local time
  tt1 is a time tuple in UTC when dt is in UTC

but then timetuple() and utctimetuple() are identical.  Do we want

  tt2 is a time tuple in UTC when dt is in localtime
  tt2 is not meaningful when dt is in UTC

?  In that case utctimetuple should be modified.


BTW the date class claims to have a utctimetuple() in its docstring,
but utctimetuple is only defined in datetime.

P.S. Would you like me to commit my prototype datetime.time
implementation to Zope3 CVS after we finish it?

Marius Gedminas
-- 
Look!  Before our very eyes, the future is becoming the past.