[bindings/python] Keep operand order in reflected GncNumeric operators

The reflected operators swapped their operands, so an int or float on
the left of a GncNumeric gave the wrong answer for the non-commutative
operations: 10 - GncNumeric(3) returned -7 and 10 / GncNumeric(4)
returned 2/5. Convert the left-hand operand and keep it on the left.

bindings/python/gnucash_core.py

@@ -625,8 +625,9 @@ class GncNumeric(GnuCashCoreClass):
         forward.__doc__ = monomorphic_operator.__doc__
 
         def reverse(b, a):
+            # b is self, the right-hand operand; a is the left-hand one.
             if isinstance(a, (GncNumeric, int, float)):
-                return forward(b, a)
+                return forward(GncNumeric(a), b)
             else:
                 return NotImplemented
         reverse.__name__ = '__r' + fallback_operator.__name__ + '__'

bindings/python/tests/test_numeric.py

@@ -85,6 +85,16 @@ class TestGncNumeric(TestCase):
         self.assertEqual(fraction.numerator, 1000)
         self.assertEqual(fraction.denominator, 3)
 
+    def test_reflected_operators(self):
+        # A plain number on the left must stay the left-hand operand.
+        self.assertEqual(10 - GncNumeric(3), GncNumeric(7))
+        self.assertEqual(1.5 - GncNumeric(1, 2), GncNumeric(1))
+        self.assertEqual(10 / GncNumeric(4), GncNumeric(5, 2))
+        self.assertEqual(1 / GncNumeric(3), GncNumeric(1, 3))
+        self.assertEqual(7 // GncNumeric(2), GncNumeric(3))
+        self.assertEqual(10 + GncNumeric(3), GncNumeric(13))
+        self.assertEqual(10 * GncNumeric(3), GncNumeric(30))
+
     def test_incorect_args(self):
         with self.assertRaises(TypeError):
             GncNumeric(1, 2, 3)