tz_isoformat needs datetime object to determine daylight saving

never pass None into tzinfo.utcoffset (doesn't make sense; it's not deterministic)

2 files changed, 8 insertions, 7 deletions

diff --git a/src/isodate/isostrf.py b/src/isodate/isostrf.py
index e492a39..34785d4 100644
--- a/src/isodate/isostrf.py
+++ b/src/isodate/isostrf.py
@@ -97,9 +97,9 @@ STRF_DT_MAP = {'%d': lambda tdt, yds: '%02d' % tdt.day,
                                    (('%%0%dd' % yds) % tdt.year),
                '%C': lambda tdt, yds: (((yds != 4) and '+') or '') +\
                                    (('%%0%dd' % (yds - 2)) % (tdt.year / 100)),
-               '%h': lambda tdt, yds: tz_isoformat(tdt.tzinfo, '%h'),
-               '%Z': lambda tdt, yds: tz_isoformat(tdt.tzinfo, '%Z'),
-               '%z': lambda tdt, yds: tz_isoformat(tdt.tzinfo, '%z'),
+               '%h': lambda tdt, yds: tz_isoformat(tdt, '%h'),
+               '%Z': lambda tdt, yds: tz_isoformat(tdt, '%Z'),
+               '%z': lambda tdt, yds: tz_isoformat(tdt, '%z'),
                '%%': lambda tdt, yds: '%'}
 
 STRF_D_MAP = {'%d': lambda tdt, yds: '%02d' % tdt.days,
diff --git a/src/isodate/isotzinfo.py b/src/isodate/isotzinfo.py
index 79797fa..df01384 100644
--- a/src/isodate/isotzinfo.py
+++ b/src/isodate/isotzinfo.py
@@ -74,7 +74,7 @@ def parse_tzinfo(tzstring):
                             int(groups['tzmin'] or 0))
     raise ISO8601Error('%s not a valid time zone info' % tzstring)
 
-def tz_isoformat(tzinfo, format='%Z'):
+def tz_isoformat(dt, format='%Z'):
     '''
     return time zone offset ISO 8601 formatted. 
     The various ISO formats can be chosen with the format parameter.
@@ -87,11 +87,12 @@ def tz_isoformat(tzinfo, format='%Z'):
         %z ... +-HHMM
         %Z ... +-HH:MM
     '''
-    if (tzinfo is None) or (tzinfo.utcoffset(None) is None):
+    tzinfo = dt.tzinfo
+    if (tzinfo is None) or (tzinfo.utcoffset(dt) is None):
         return ''
-    if tzinfo.utcoffset(None) == ZERO and tzinfo.dst(None) == ZERO:
+    if tzinfo.utcoffset(dt) == ZERO and tzinfo.dst(dt) == ZERO:
         return 'Z'
-    tdelta = tzinfo.utcoffset(None)
+    tdelta = tzinfo.utcoffset(dt)
     seconds = tdelta.days * 24 * 60 * 60 + tdelta.seconds
     sign = ((seconds < 0) and '-') or '+'
     seconds = abs(seconds)