Give precedence to '__conform__()' over superclasses choosing adapter

Close #456

NEWS

@@ -26,6 +26,8 @@ New features:
 - Added `~psycopg2.extensions.quote_ident()` function (:ticket:`#359`).
 - Added `~connection.get_dsn_parameters()` connection method (:ticket:`#364`).
 - `~cursor.callproc()` now accepts a dictionary of parameters (:ticket:`#381`).
+- Give precedence to `!__conform__()` over superclasses to choose an object
+  adapter (:ticket:`#456`).
 - Using Python C API decoding functions and codecs caching for faster
   unicode encoding/decoding (:ticket:`#473`).
 - `~cursor.executemany()` slowness addressed by

psycopg/microprotocols.c

@@ -153,15 +153,6 @@ microprotocols_adapt(PyObject *obj, PyObject *proto, PyObject *alt)
         return adapted;
     }
 
-    /* Check if a superclass can be adapted and use the same adapter. */
-    if (!(adapter = _get_superclass_adapter(obj, proto))) {
-        return NULL;
-    }
-    if (Py_None != adapter) {
-        adapted = PyObject_CallFunctionObjArgs(adapter, obj, NULL);
-        return adapted;
-    }
-
     /* try to have the protocol adapt this object*/
     if ((meth = PyObject_GetAttrString(proto, "__adapt__"))) {
         adapted = PyObject_CallFunctionObjArgs(meth, obj, NULL);
@@ -181,7 +172,7 @@ microprotocols_adapt(PyObject *obj, PyObject *proto, PyObject *alt)
         PyErr_Clear();
     }
 
-    /* and finally try to have the object adapt itself */
+    /* then try to have the object adapt itself */
     if ((meth = PyObject_GetAttrString(obj, "__conform__"))) {
         adapted = PyObject_CallFunctionObjArgs(meth, proto, NULL);
         Py_DECREF(meth);
@@ -200,6 +191,15 @@ microprotocols_adapt(PyObject *obj, PyObject *proto, PyObject *alt)
         PyErr_Clear();
     }
 
+    /* Finally check if a superclass can be adapted and use the same adapter. */
+    if (!(adapter = _get_superclass_adapter(obj, proto))) {
+        return NULL;
+    }
+    if (Py_None != adapter) {
+        adapted = PyObject_CallFunctionObjArgs(adapter, obj, NULL);
+        return adapted;
+    }
+
     /* else set the right exception and return NULL */
     PyOS_snprintf(buffer, 255, "can't adapt type '%s'",
         Py_TYPE(obj)->tp_name);

tests/test_types_basic.py

@@ -422,6 +422,19 @@ class AdaptSubclassTest(unittest.TestCase):
         finally:
             del psycopg2.extensions.adapters[A, psycopg2.extensions.ISQLQuote]
 
+    def test_conform_subclass_precedence(self):
+
+        import psycopg2.extensions as ext
+
+        class foo(tuple):
+            def __conform__(self, proto):
+                return self
+
+            def getquoted(self):
+                return 'bar'
+
+        self.assertEqual(ext.adapt(foo((1, 2, 3))).getquoted(), 'bar')
+
 
 class ByteaParserTest(unittest.TestCase):
     """Unit test for our bytea format parser."""