I was playing with python in the Houdini master 10.0.465 and found the following:
1. Create a grid ( or whatever) object
2. Add the Transform surface node to it
3. In the rotate ( transform, scale …) parameter field put
hscriptExpression(“$F”). The Python button is turned on
4. Try to animate
You will see that nothing is happening. Although the value of $F does show up in the field. Once you change the python expression back to hscript
( hscriptExpression(“$F”) -> $F ) everything is okay now.
Is this a bug?

http://www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=18039&highlight=python [sidefx.com]

Thank you Swann!!!