date and datetime comparison: when we don't know how to

compare against "the other" argument, we raise TypeError,
in order to prevent comparison from falling back to the
default (and worse than useless, in this case) comparison
by object address.

That's fine so far as it goes, but leaves no way for
another date/datetime object to make itself comparable
to our objects.  For example, it leaves Marc-Andre no way
to teach mxDateTime dates how to compare against Python
dates.

Discussion on Python-Dev raised a number of impractical
ideas, and the simple one implemented here:  when we don't
know how to compare against "the other" argument, we raise
TypeError *unless* the other object has a timetuple attr.
In that case, we return NotImplemented instead, and Python
will give the other object a shot at handling the
comparison then.

Note that comparisons of time and timedelta objects still
suffer the original problem, though.

1 files changed, 38 insertions, 0 deletions

diff --git a/Lib/test/test_datetime.py b/Lib/test/test_datetime.py
index 0b9597a..995b6a0 100644
--- a/Lib/test/test_datetime.py
+++ b/Lib/test/test_datetime.py
@@ -880,6 +880,44 @@ class TestDate(unittest.TestCase):
             self.assertRaises(TypeError, lambda: badarg > t1)
             self.assertRaises(TypeError, lambda: badarg >= t1)
 
+    def test_mixed_compare(self):
+        our = self.theclass(2000, 4, 5)
+        self.assertRaises(TypeError, cmp, our, 1)
+        self.assertRaises(TypeError, cmp, 1, our)
+
+        class AnotherDateTimeClass(object):
+            def __cmp__(self, other):
+                # Return "equal" so calling this can't be confused with
+                # compare-by-address (which never says "equal" for distinct
+                # objects).
+                return 0
+
+        # This still errors, because date and datetime comparison raise
+        # TypeError instead of NotImplemented when they don't know what to
+        # do, in order to stop comparison from falling back to the default
+        # compare-by-address.
+        their = AnotherDateTimeClass()
+        self.assertRaises(TypeError, cmp, our, their)
+        # Oops:  The next stab raises TypeError in the C implementation,
+        # but not in the Python implementation of datetime.  The difference
+        # is due to that the Python implementation defines __cmp__ but
+        # the C implementation defines tp_richcompare.  This is more pain
+        # to fix than it's worth, so commenting out the test.
+        # self.assertEqual(cmp(their, our), 0)
+
+        # But date and datetime comparison return NotImplemented instead if the
+        # other object has a timetuple attr.  This gives the other object a
+        # chance to do the comparison.
+        class Comparable(AnotherDateTimeClass):
+            def timetuple(self):
+                return ()
+
+        their = Comparable()
+        self.assertEqual(cmp(our, their), 0)
+        self.assertEqual(cmp(their, our), 0)
+        self.failUnless(our == their)
+        self.failUnless(their == our)
+
     def test_bool(self):
         # All dates are considered true.
         self.failUnless(self.theclass.min)