Provide cursor.description as named tuple if possible

If namedtuple() is not available, use regular tuples.

NEWS

@@ -11,6 +11,7 @@ What's new in psycopg 2.4
     into Python tuples/namedtuples.
   - More efficient iteration on named cursors, fetching 'itersize' records at
     time from the backend.
+  - 'cursor.description' is provided in named tuples if available.
   - Connections and cursors are weakly referenceable.
   - Added 'b' and 't' mode to large objects: write can deal with both bytes
     strings and unicode; read can return either bytes strings or decoded

doc/src/cursor.rst

@@ -39,8 +39,9 @@ The ``cursor`` class
 
         This read-only attribute is a sequence of 7-item sequences.  
 
-        Each of these sequences contains information describing one result
-        column: 
+        Each of these sequences is a named tuple (a regular tuple if
+        `!collections.namedtuple()` is not available) containing information
+        describing one result column:
 
         0.  `!name`: the name of the column returned.
         1.  `!type_code`: the PostgreSQL OID of the column. You can use the
@@ -53,11 +54,11 @@ The ``cursor`` class
             always `!None` unless the :envvar:`PSYCOPG_DISPLAY_SIZE` parameter
             is set at compile time. See also PQgetlength_.
         3.  `!internal_size`: the size in bytes of the column associated to
-            this column on the server. Set to a egative value for
+            this column on the server. Set to a negative value for
             variable-size types See also PQfsize_.
         4.  `!precision`: total number of significant digits in columns of
             type |NUMERIC|_. `!None` for other types.
-        5.  `!scale`: count of decimal digits in the freactional part in
+        5.  `!scale`: count of decimal digits in the fractional part in
             columns of type |NUMERIC|. `!None` for other types.
         6.  `!null_ok`: always `!None` as not easy to retrieve from the libpq.
 
@@ -72,6 +73,10 @@ The ``cursor`` class
         .. _NUMERIC: http://www.postgresql.org/docs/9.0/static/datatype-numeric.html#DATATYPE-NUMERIC-DECIMAL
         .. |NUMERIC| replace:: :sql:`NUMERIC`
 
+        .. versionchanged:: 2.4
+            if possible, columns descriptions are named tuple instead of
+            regular tuples.
+
     .. method:: close()
           
         Close the cursor now (rather than whenever `!__del__()` is

psycopg/pqpath.c

@@ -42,6 +42,9 @@
 #include <string.h>
 
 
+extern HIDDEN PyObject *psyco_DescriptionType;
+
+
 /* Strip off the severity from a Postgres error message. */
 static const char *
 strip_severity(const char *msg)
@@ -948,7 +951,6 @@ _pq_fetch_tuples(cursorObject *curs)
         Py_BLOCK_THREADS;
 
         dtitem = PyTuple_New(7);
-        PyTuple_SET_ITEM(curs->description, i, dtitem);
 
         /* fill the right cast function by accessing three different dictionaries:
            - the per-cursor dictionary, if available (can be NULL or None)
@@ -1022,6 +1024,22 @@ _pq_fetch_tuples(cursorObject *curs)
         Py_INCREF(Py_None);
         PyTuple_SET_ITEM(dtitem, 6, Py_None);
 
+        /* Convert into a namedtuple if available */
+        if (Py_None != psyco_DescriptionType) {
+            PyObject *tmp = dtitem;
+            if ((dtitem = PyObject_CallObject(psyco_DescriptionType, tmp))) {
+                Py_DECREF(tmp);
+            }
+            else {
+                /* FIXME: this function is painfully missing any error check.
+                 * The caller doesn't expect them, so swallow it. */
+                PyErr_Clear();
+                dtitem = tmp;
+            }
+        }
+
+        PyTuple_SET_ITEM(curs->description, i, dtitem);
+
         Py_UNBLOCK_THREADS;
     }
 

psycopg/psycopgmodule.c

@@ -69,6 +69,9 @@ HIDDEN int psycopg_debug_enabled = 0;
 /* Python representation of SQL NULL */
 HIDDEN PyObject *psyco_null = NULL;
 
+/* The type of the cursor.description items */
+HIDDEN PyObject *psyco_DescriptionType = NULL;
+
 /** connect module-level function **/
 #define psyco_connect_doc \
 "connect(dsn, ...) -- Create a new database connection.\n\n"               \
@@ -685,6 +688,44 @@ psyco_GetDecimalType(void)
 }
 
 
+/* Create a namedtuple for cursor.description items
+ *
+ * Return None in case of expected errors (e.g. namedtuples not available)
+ * NULL in case of errors to propagate.
+ */
+static PyObject *
+psyco_make_description_type(void)
+{
+    PyObject *nt = NULL;
+    PyObject *coll = NULL;
+    PyObject *rv = NULL;
+
+    /* Try to import collections.namedtuple */
+    if (!(coll = PyImport_ImportModule("collections"))) {
+        Dprintf("psyco_make_description_type: collections import failed");
+        PyErr_Clear();
+        rv = Py_None;
+        goto exit;
+    }
+    if (!(nt = PyObject_GetAttrString(coll, "namedtuple"))) {
+        Dprintf("psyco_make_description_type: no collections.namedtuple");
+        PyErr_Clear();
+        rv = Py_None;
+        goto exit;
+    }
+
+    /* Build the namedtuple */
+    rv = PyObject_CallFunction(nt, "ss", "Column",
+        "name type_code display_size internal_size precision scale null_ok");
+
+exit:
+    Py_XDECREF(coll);
+    Py_XDECREF(nt);
+
+    return rv;
+}
+
+
 /** method table and module initialization **/
 
 static PyMethodDef psycopgMethods[] = {
@@ -886,6 +927,7 @@ INIT_MODULE(_psycopg)(void)
     psycoEncodings = PyDict_New();
     psyco_encodings_fill(psycoEncodings);
     psyco_null = Bytes_FromString("NULL");
+    psyco_DescriptionType = psyco_make_description_type();
 
     /* set some module's parameters */
     PyModule_AddStringConstant(module, "__version__", PSYCOPG_VERSION);

tests/extras_dictcursor.py

@@ -16,7 +16,7 @@
 
 import psycopg2
 import psycopg2.extras
-from testutils import unittest
+from testutils import unittest, skip_if_no_namedtuple
 from testconfig import dsn
 
 
@@ -112,18 +112,6 @@ class ExtrasDictCursorTests(unittest.TestCase):
         self.failUnless(row[0] == 'qux')
 
 
-def if_has_namedtuple(f):
-    def if_has_namedtuple_(self):
-        try:
-            from collections import namedtuple
-        except ImportError:
-            return self.skipTest("collections.namedtuple not available")
-        else:
-            return f(self)
-
-    if_has_namedtuple_.__name__ = f.__name__
-    return if_has_namedtuple_
-
 class NamedTupleCursorTest(unittest.TestCase):
     def setUp(self):
         from psycopg2.extras import NamedTupleConnection
@@ -147,7 +135,7 @@ class NamedTupleCursorTest(unittest.TestCase):
         if self.conn is not None:
             self.conn.close()
 
-    @if_has_namedtuple
+    @skip_if_no_namedtuple
     def test_fetchone(self):
         curs = self.conn.cursor()
         curs.execute("select * from nttest where i = 1")
@@ -157,7 +145,7 @@ class NamedTupleCursorTest(unittest.TestCase):
         self.assertEqual(t[1], 'foo')
         self.assertEqual(t.s, 'foo')
 
-    @if_has_namedtuple
+    @skip_if_no_namedtuple
     def test_fetchmany(self):
         curs = self.conn.cursor()
         curs.execute("select * from nttest order by 1")
@@ -168,7 +156,7 @@ class NamedTupleCursorTest(unittest.TestCase):
         self.assertEqual(res[1].i, 2)
         self.assertEqual(res[1].s, 'bar')
 
-    @if_has_namedtuple
+    @skip_if_no_namedtuple
     def test_fetchall(self):
         curs = self.conn.cursor()
         curs.execute("select * from nttest order by 1")
@@ -181,7 +169,7 @@ class NamedTupleCursorTest(unittest.TestCase):
         self.assertEqual(res[2].i, 3)
         self.assertEqual(res[2].s, 'baz')
 
-    @if_has_namedtuple
+    @skip_if_no_namedtuple
     def test_iter(self):
         curs = self.conn.cursor()
         curs.execute("select * from nttest order by 1")
@@ -219,7 +207,7 @@ class NamedTupleCursorTest(unittest.TestCase):
             # skip the test
             pass
 
-    @if_has_namedtuple
+    @skip_if_no_namedtuple
     def test_record_updated(self):
         curs = self.conn.cursor()
         curs.execute("select 1 as foo;")
@@ -231,7 +219,7 @@ class NamedTupleCursorTest(unittest.TestCase):
         self.assertEqual(r.bar, 2)
         self.assertRaises(AttributeError, getattr, r, 'foo')
 
-    @if_has_namedtuple
+    @skip_if_no_namedtuple
     def test_no_result_no_surprise(self):
         curs = self.conn.cursor()
         curs.execute("update nttest set s = s")
@@ -240,7 +228,7 @@ class NamedTupleCursorTest(unittest.TestCase):
         curs.execute("update nttest set s = s")
         self.assertRaises(psycopg2.ProgrammingError, curs.fetchall)
 
-    @if_has_namedtuple
+    @skip_if_no_namedtuple
     def test_minimal_generation(self):
         # Instrument the class to verify it gets called the minimum number of times.
         from psycopg2.extras import NamedTupleCursor

tests/test_cursor.py

@@ -27,7 +27,7 @@ import psycopg2
 import psycopg2.extensions
 from psycopg2.extensions import b
 from testconfig import dsn
-from testutils import unittest, skip_before_postgres
+from testutils import unittest, skip_before_postgres, skip_if_no_namedtuple
 
 class CursorTests(unittest.TestCase):
 
@@ -172,6 +172,42 @@ class CursorTests(unittest.TestCase):
         # everything swallowed in two gulps
         self.assertEqual(rv, [(i,((i - 1) % 30) + 1) for i in range(1,51)])
 
+    @skip_if_no_namedtuple
+    def test_namedtuple_description(self):
+        curs = self.conn.cursor()
+        curs.execute("""select
+            3.14::decimal(10,2) as pi,
+            'hello'::text as hi,
+            '2010-02-18'::date as now;
+            """)
+        self.assertEqual(len(curs.description), 3)
+        for c in curs.description:
+            self.assertEqual(len(c), 7)  # DBAPI happy
+            for a in ('name', 'type_code', 'display_size', 'internal_size',
+                    'precision', 'scale', 'null_ok'):
+                self.assert_(hasattr(c, a), a)
+
+        c = curs.description[0]
+        self.assertEqual(c.name, 'pi')
+        self.assert_(c.type_code in psycopg2.extensions.DECIMAL.values)
+        self.assert_(c.internal_size > 0)
+        self.assertEqual(c.precision, 10)
+        self.assertEqual(c.scale, 2)
+
+        c = curs.description[1]
+        self.assertEqual(c.name, 'hi')
+        self.assert_(c.type_code in psycopg2.STRING.values)
+        self.assert_(c.internal_size < 0)
+        self.assertEqual(c.precision, None)
+        self.assertEqual(c.scale, None)
+
+        c = curs.description[2]
+        self.assertEqual(c.name, 'now')
+        self.assert_(c.type_code in psycopg2.extensions.DATE.values)
+        self.assert_(c.internal_size > 0)
+        self.assertEqual(c.precision, None)
+        self.assertEqual(c.scale, None)
+
 
 def test_suite():
     return unittest.TestLoader().loadTestsFromName(__name__)

tests/testutils.py

@@ -127,6 +127,19 @@ def skip_if_tpc_disabled(f):
     return skip_if_tpc_disabled_
 
 
+def skip_if_no_namedtuple(f):
+    def skip_if_no_namedtuple_(self):
+        try:
+            from collections import namedtuple
+        except ImportError:
+            return self.skipTest("collections.namedtuple not available")
+        else:
+            return f(self)
+
+    skip_if_no_namedtuple_.__name__ = f.__name__
+    return skip_if_no_namedtuple_
+
+
 def skip_if_no_iobase(f):
     """Skip a test if io.TextIOBase is not available."""
     def skip_if_no_iobase_(self):