Added tests to verify the new Diagnostics properties

diag can be used on exceptions raised without cursor and is independent from
the cursor.

Docs updated to reflect the changes.

doc/src/extensions.rst

@@ -165,9 +165,9 @@ functionalities defined by the |DBAPI|_.
         table_name
 
         A string with the error field if available; `!None` if not available.
-        The attribute value is available only if the error sent by the server
-        includes the specified field and should remain available until the
-        cursor that generated the exception executes another query.
+        The attribute value is available only if the error sent by the server:
+        not all the fields are available for all the errors and for all the
+        server versions.
 
 
 .. autofunction:: set_wait_callback(f)

tests/test_module.py

@@ -221,6 +221,38 @@ class ExceptionsTestCase(unittest.TestCase):
 
         self.assertEqual(diag.sqlstate, '42P01')
 
+    def test_diagnostics_independent(self):
+        cur = self.conn.cursor()
+        try:
+            cur.execute("l'acqua e' poca e 'a papera nun galleggia")
+        except Exception, exc:
+            diag1 = exc.diag
+
+        self.conn.rollback()
+
+        try:
+            cur.execute("select level from water where ducks > 1")
+        except psycopg2.Error, exc:
+            diag2 = exc.diag
+
+        self.assertEqual(diag1.sqlstate, '42601')
+        self.assertEqual(diag2.sqlstate, '42P01')
+
+    def test_diagnostics_from_commit(self):
+        cur = self.conn.cursor()
+        cur.execute("""
+            create temp table test_deferred (
+               data int primary key,
+               ref int references test_deferred (data)
+                   deferrable initially deferred)
+        """)
+        cur.execute("insert into test_deferred values (1,2)")
+        try:
+            self.conn.commit()
+        except psycopg2.Error, exc:
+            e = exc
+        self.assertEqual(e.diag.sqlstate, '23503')
+
     @skip_before_postgres(9, 3)
     def test_9_3_diagnostics(self):
         cur = self.conn.cursor()