Fix plistlib loading of partial ISO 8601 dates

plistlib's date regex makes the month, day and time components optional,
but _date_from_string stopped building the datetime arguments at the
first missing component, so a partial date such as ``2024-06Z`` raised a
confusing TypeError instead of producing a datetime. Default the omitted
components to the start of the period.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: JavaZeroo

Lib/plistlib.py

@@ -139,13 +139,11 @@ def _decode_base64(s):
 
 def _date_from_string(s, aware_datetime):
     order = ('year', 'month', 'day', 'hour', 'minute', 'second')
+    # Smaller units may be omitted; default them to the start of the period.
+    defaults = (1, 1, 1, 0, 0, 0)
     gd = _dateParser.match(s).groupdict()
-    lst = []
-    for key in order:
-        val = gd[key]
-        if val is None:
-            break
-        lst.append(int(val))
+    lst = [int(val) if (val := gd[key]) is not None else default
+           for key, default in zip(order, defaults)]
     if aware_datetime:
         return datetime.datetime(*lst, tzinfo=datetime.UTC)
     return datetime.datetime(*lst)

Lib/test/test_plistlib.py

@@ -938,6 +938,24 @@ def test_load_aware_datetime(self):
                             aware_datetime=True)
         self.assertEqual(dt.tzinfo, datetime.UTC)
 
+    def test_load_partial_datetime(self):
+        # Smaller units may be omitted; missing components default to the
+        # start of the period.
+        for data, expected in [
+            (b"<plist><date>2024Z</date></plist>",
+             datetime.datetime(2024, 1, 1)),
+            (b"<plist><date>2024-06Z</date></plist>",
+             datetime.datetime(2024, 6, 1)),
+            (b"<plist><date>2024-06-07Z</date></plist>",
+             datetime.datetime(2024, 6, 7)),
+            (b"<plist><date>2024-06-07T08Z</date></plist>",
+             datetime.datetime(2024, 6, 7, 8)),
+            (b"<plist><date>2024-06-07T08:09Z</date></plist>",
+             datetime.datetime(2024, 6, 7, 8, 9)),
+        ]:
+            with self.subTest(data=data):
+                self.assertEqual(plistlib.loads(data), expected)
+
     @unittest.skipUnless("America/Los_Angeles" in zoneinfo.available_timezones(),
                          "Can't find timezone datebase")
     def test_dump_aware_datetime(self):