Column objects can be sliced

Close #1034.

NEWS

@@ -8,6 +8,8 @@ What's new in psycopg 2.8.5
   (:ticket:`#1019`).
 - Added support for `~logging.LoggerAdapter` in
   `~psycopg2.extras.LoggingConnection` (:ticket:`#1026`).
+- `~psycopg2.extensions.Column` objects in `cursor.description` can be sliced
+  (:ticket:`#1034`).
 
 
 What's new in psycopg 2.8.4

psycopg/column_type.c

@@ -233,6 +233,32 @@ column_getitem(columnObject *self, Py_ssize_t item)
 }
 
 
+static PyObject*
+column_subscript(columnObject* self, PyObject* item)
+{
+    PyObject *t = NULL;
+    PyObject *rv = NULL;
+
+    /* t = tuple(self) */
+    if (!(t = PyObject_CallFunctionObjArgs(
+            (PyObject *)&PyTuple_Type, (PyObject *)self, NULL))) {
+        goto exit;
+    }
+
+    /* rv = t[item] */
+    rv = PyObject_GetItem(t, item);
+
+exit:
+    Py_XDECREF(t);
+    return rv;
+}
+
+static PyMappingMethods column_mapping = {
+    (lenfunc)column_len,            /* mp_length */
+    (binaryfunc)column_subscript,   /* mp_subscript */
+    0                               /* mp_ass_subscript */
+};
+
 static PySequenceMethods column_sequence = {
     (lenfunc)column_len,       /* sq_length */
     0,                         /* sq_concat */
@@ -346,7 +372,7 @@ PyTypeObject columnType = {
     (reprfunc)column_repr, /*tp_repr*/
     0,          /*tp_as_number*/
     &column_sequence, /*tp_as_sequence*/
-    0,          /*tp_as_mapping*/
+    &column_mapping,  /*tp_as_mapping*/
     0,          /*tp_hash */
     0,          /*tp_call*/
     0,          /*tp_str*/

tests/test_cursor.py

@@ -433,6 +433,11 @@ class CursorTests(ConnectingTestCase):
         self.assertEqual(curs.description[2].table_oid, None)
         self.assertEqual(curs.description[2].table_column, None)
 
+    def test_description_slice(self):
+        curs = self.conn.cursor()
+        curs.execute("select 1::int as a")
+        self.assertEqual(curs.description[0][0:2], ('a', 23))
+
     def test_pickle_description(self):
         curs = self.conn.cursor()
         curs.execute('SELECT 1 AS foo')