Don't convert '{}'::unknown into an empty list

Close #506.

NEWS

@@ -55,6 +55,8 @@ Other changes:
   connection.
 - `~connection.set_isolation_level()` will throw an exception if executed
   inside a transaction; previously it would have silently rolled it back.
+- Empty arrays no more converted into lists if they don't have a type attached
+  (:ticket:`#506`)
 
 
 What's new in psycopg 2.6.3

psycopg/typecast.c

@@ -191,14 +191,6 @@ typecast_UNKNOWN_cast(const char *str, Py_ssize_t len, PyObject *curs)
     Dprintf("typecast_UNKNOWN_cast: str = '%s',"
             " len = " FORMAT_CODE_PY_SSIZE_T, str, len);
 
-    // PostgreSQL returns {} for empty array without explicit type. We convert
-    // that to list in order to handle empty lists.
-    if (len == 2 && str[0] == '{' && str[1] == '}') {
-        return PyList_New(0);
-    }
-
-    Dprintf("typecast_UNKNOWN_cast: fallback to default cast");
-
     return typecast_default.cast(str, len, curs);
 }
 

tests/test_types_basic.py

@@ -166,12 +166,14 @@ class TypesBasicTests(ConnectingTestCase):
         curs.execute("select col from array_test where id = 2")
         self.assertEqual(curs.fetchone()[0], [])
 
-    def testEmptyArray(self):
+    def testEmptyArrayNoCast(self):
         s = self.execute("SELECT '{}' AS foo")
-        self.failUnlessEqual(s, [])
+        self.assertEqual(s, '{}')
-        s = self.execute("SELECT '{}'::text[] AS foo")
-        self.failUnlessEqual(s, [])
         s = self.execute("SELECT %s AS foo", ([],))
+        self.assertEqual(s, '{}')
+
+    def testEmptyArray(self):
+        s = self.execute("SELECT '{}'::text[] AS foo")
         self.failUnlessEqual(s, [])
         s = self.execute("SELECT 1 != ALL(%s)", ([],))
         self.failUnlessEqual(s, True)