[formatter] fix local DST datetime offsets for ':O'

'O' would get the *current* local UTC offset and apply it to all
'datetime' objects it gets applied to.
This would result in a wrong offset if the current offset includes
DST and the target 'datetime' does not or vice-versa.

'O' now determines the correct local UTC offset while respecting DST for
each individual 'datetime'.

gallery_dl/formatter.py

@@ -375,18 +375,18 @@ def _parse_offset(format_spec, default):
     fmt = _build_format_func(format_spec, default)
 
     if not offset or offset == "local":
-        is_dst = time.daylight and time.localtime().tm_isdst > 0
-        offset = -(time.altzone if is_dst else time.timezone)
+        def off(dt):
+            local = time.localtime(util.datetime_to_timestamp(dt))
+            return fmt(dt + datetime.timedelta(0, local.tm_gmtoff))
     else:
         hours, _, minutes = offset.partition(":")
         offset = 3600 * int(hours)
         if minutes:
             offset += 60 * (int(minutes) if offset > 0 else -int(minutes))
+        offset = datetime.timedelta(0, offset)
 
-    offset = datetime.timedelta(seconds=offset)
-
-    def off(obj):
-        return fmt(obj + offset)
+        def off(obj):
+            return fmt(obj + offset)
     return off
 
 

test/test_formatter.py

@@ -31,8 +31,9 @@ class TestFormatter(unittest.TestCase):
         "h": "<p>foo </p> &amp; bar <p> </p>",
         "u": "&#x27;&lt; / &gt;&#x27;",
         "t": 1262304000,
-        "dt": datetime.datetime(2010, 1, 1),
         "ds": "2010-01-01T01:00:00+0100",
+        "dt": datetime.datetime(2010, 1, 1),
+        "dt_dst": datetime.datetime(2010, 6, 1),
         "name": "Name",
         "title1": "Title",
         "title2": "",
@@ -236,19 +237,18 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{ds:D%Y-%m-%dT%H:%M:%S%z/O1}", "2010-01-01 01:00:00")
         self._run_test("{t!d:O2}", "2010-01-01 02:00:00")
 
-        orig_daylight = time.daylight
-        orig_timezone = time.timezone
-        orig_altzone = time.altzone
-        try:
-            time.daylight = False
-            time.timezone = -3600
-            self._run_test("{dt:O}", "2010-01-01 01:00:00")
-            time.timezone = 7200
-            self._run_test("{dt:Olocal}", "2009-12-31 22:00:00")
-        finally:
-            time.daylight = orig_daylight
-            time.timezone = orig_timezone
-            time.altzone = orig_altzone
+    def test_offset_local(self):
+        ts = self.kwdict["dt"].replace(tzinfo=datetime.UTC).timestamp()
+        offset = time.localtime(ts).tm_gmtoff
+        dt = self.kwdict["dt"] + datetime.timedelta(seconds=offset)
+        self._run_test("{dt:O}", str(dt))
+        self._run_test("{dt:Olocal}", str(dt))
+
+        ts = self.kwdict["dt_dst"].replace(tzinfo=datetime.UTC).timestamp()
+        offset = time.localtime(ts).tm_gmtoff
+        dt = self.kwdict["dt_dst"] + datetime.timedelta(seconds=offset)
+        self._run_test("{dt_dst:O}", str(dt))
+        self._run_test("{dt_dst:Olocal}", str(dt))
 
     def test_sort(self):
         self._run_test("{l:S}" , "['a', 'b', 'c']")