testcode for python.txt

doc/internal/python.txt

@@ -37,11 +37,15 @@ Theano doesn't use your grandfather's python.
   * functions (function objects) can have attributes too. This technique
     is often used to define a function's error messages.
 
-    .. code-block:: python
+    .. testcode::
 
         def f(): return f.a
         f.a = 5
-        f() # returns 5
+        f() 
+        
+    .. testoutput::
+    
+        5
 
   * Warning about mutual imports: