Fixed adaptation of lists of empty lists

...somehow. Postgres doesn't support them and converts them into a
simple empty array. However this is not really our concern: the syntax
we return is valid.

Close #788

NEWS

@@ -8,6 +8,7 @@ What's new in psycopg 2.7.6
   (:ticket:`#746`).
 - Fixed building on modern FreeBSD versions with Python 3.7 (:ticket:`#755`).
 - Fixed hang trying to :sql:`COPY` via `~cursor.execute()` (:ticket:`#781`).
+- Fixed adaptation of arrays of empty arrays (:ticket:`#788`).
 - Fixed segfault accessing the `connection.readonly` and
   `connection.deferrable` repeatedly (:ticket:`#790`).
 - `~psycopg2.extras.execute_values()` accepts `~psycopg2.sql.Composable`

psycopg/adapter_list.c

@@ -55,6 +55,8 @@ list_quote(listObject *self)
     /* empty arrays are converted to NULLs (still searching for a way to
        insert an empty array in postgresql */
     if (len == 0) {
+        /* it cannot be ARRAY[] because it would make empty lists unusable
+         * in any() without a cast. But we may convert it into ARRAY[] below */
         res = Bytes_FromString("'{}'");
         goto exit;
     }
@@ -79,7 +81,19 @@ list_quote(listObject *self)
 
             /* Lists of arrays containing only nulls are also not supported
              * by the ARRAY construct so we should do some special casing */
-            if (!PyList_Check(wrapped) || Bytes_AS_STRING(qs[i])[0] == 'A') {
+            if (PyList_Check(wrapped)) {
+                if (Bytes_AS_STRING(qs[i])[0] == 'A') {
+                    all_nulls = 0;
+                }
+                else if (0 == strcmp(Bytes_AS_STRING(qs[i]), "'{}'")) {
+                    /* case of issue #788: '{{}}' is not supported but
+                     * array[array[]] is */
+                    all_nulls = 0;
+                    Py_CLEAR(qs[i]);
+                    qs[i] = Bytes_FromString("ARRAY[]");
+                }
+            }
+            else {
                 all_nulls = 0;
             }
         }

tests/test_types_basic.py

@@ -166,6 +166,13 @@ class TypesBasicTests(ConnectingTestCase):
         curs.execute("select col from array_test where id = 2")
         self.assertEqual(curs.fetchone()[0], [])
 
+    @testutils.skip_before_postgres(8, 4)
+    def testNestedEmptyArray(self):
+        # issue #788
+        curs = self.conn.cursor()
+        curs.execute("select 10 = any(%s::int[])", ([[]], ))
+        self.assertFalse(curs.fetchone()[0])
+
     def testEmptyArrayNoCast(self):
         s = self.execute("SELECT '{}' AS foo")
         self.assertEqual(s, '{}')