Support a <null> type column

A TypeObject is needed instead of returning None, as None is used to
verify that the type returned is in the TYPEMAP.

E.g.,

  cursor.execute('select NULL from DUAL')

fails without this change, as well as situations where a user might
explicitly return NULL in place of an actual value.

Add a test.

Author: butson

pynuodb/datatype.py

@@ -154,8 +154,9 @@ def __cmp__(self, other):
 NUMBER = TypeObject(int, decimal.Decimal)
 DATETIME = TypeObject(Timestamp, Date, Time)
 ROWID = TypeObject()
+NULL = TypeObject(None)
 
-TYPEMAP = {"<null>": None,
+TYPEMAP = {"<null>": NULL,
            "string": STRING,
            "char": STRING,
            "varchar": STRING,

tests/nuodb_types_test.py

@@ -112,3 +112,16 @@ def test_datetime_types(self):
         assert row[1] == datetime.time(5, 44, 33, 221100)
         assert row[2] == datetime.datetime(2000, 1, 1, 5, 44, 33, 221100)
         assert row[3] == datetime.datetime(2000, 1, 1, 5, 44, 33, 221100)
+
+    def test_null_type(self):
+        con = self._connect()
+        cursor = con.cursor()
+
+        null_type = self.driver.TypeObjectFromNuodb('<null>')
+
+        cursor.execute("SELECT NULL from dual")
+        row = cursor.fetchone()
+
+        assert len(row) == 1
+        assert cursor.description[0][1] == null_type
+        assert row[0] is None