auto-execute python on a node

[tagosaku]

HI, I am learning python script. So far, I can execute python code by clicking a custom button, and it sets parameters of today(string) and switch(integer). 

I wonder how we can automatically execute that python code. For instance,  when I select the node SOP, or houdini accesses node network from somewhere as need, these parameter values are automatically calculated or updated.  

 

Question 1
When I directly write python code in the parameter like second screenshot, it works for integer but not strings.  Why did not the string work? 

Question 2
Let's say I have 20 parameters, and ideally I want to control all the parameters from one master python script in the code parameter, instead of writing python code each parameter 20 times.  Then I was thinking :
code parameter{string) > Callback script > exec(kwargs['node'].parm('code').eval())
However, auto-execution happens only when I change code's strings. And this is not what I am looking for. Execution should happen when I select the node or Houdini access it during  node network calculation. 

If anyone knows how I should change, it would be appreciated. Thanks!