Cache FixedOffsetTimezone instances

Avoid creating new a new FixedOffsetTimezone instance if one with the
same offset and name has been created before. This will save memory
when returning many rows containing "timestamp with timezone" columns,
and also improves comparability.

lib/tz.py

@@ -38,17 +38,36 @@ class FixedOffsetTimezone(datetime.tzinfo):
     with a small change to the `!__init__()` method to allow for pickling
     and a default name in the form ``sHH:MM`` (``s`` is the sign.).
 
+    The implementation also caches instances. During creation, if a
+    FixedOffsetTimezone instance has previously been created with the same
+    offset and name that instance will be returned. This saves memory and
+    improves comparability.
+
     .. __: http://docs.python.org/library/datetime.html#datetime-tzinfo
     """
     _name = None
     _offset = ZERO
 
+    _cache = {}
+
     def __init__(self, offset=None, name=None):
         if offset is not None:
             self._offset = datetime.timedelta(minutes = offset)
         if name is not None:
             self._name = name
 
+    def __new__(cls, offset=None, name=None):
+        """Return a suitable instance created earlier if it exists
+        """
+        key = (offset, name)
+        try:
+            return cls._cache[key]
+        except KeyError:
+            tz = datetime.tzinfo.__new__(cls, offset, name)
+            tz.__init__(offset, name)
+            cls._cache[key] = tz
+            return tz
+
     def __repr__(self):
         offset_mins = self._offset.seconds // 60 + self._offset.days * 24 * 60
         return "psycopg2.tz.FixedOffsetTimezone(offset=%r, name=%r)" \

tests/test_dates.py

@@ -532,6 +532,13 @@ class FixedOffsetTimezoneTests(unittest.TestCase):
         tzinfo = FixedOffsetTimezone(name="FOO")
         self.assertEqual(repr(tzinfo), "psycopg2.tz.FixedOffsetTimezone(offset=0, name='FOO')")
 
+    def test_instance_caching(self):
+        self.assert_(FixedOffsetTimezone(name="FOO") is FixedOffsetTimezone(name="FOO"))
+        self.assert_(FixedOffsetTimezone(7 * 60) is FixedOffsetTimezone(7 * 60))
+        self.assert_(FixedOffsetTimezone(-9 * 60, 'FOO') is FixedOffsetTimezone(-9 * 60, 'FOO'))
+        self.assert_(FixedOffsetTimezone(9 * 60) is not FixedOffsetTimezone(9 * 60, 'FOO'))
+        self.assert_(FixedOffsetTimezone(name='FOO') is not FixedOffsetTimezone(9 * 60, 'FOO'))
+
 def test_suite():
     return unittest.TestLoader().loadTestsFromName(__name__)