I have searched a lot and I know how to open a directory dialog window. 
But what I am looking for is the method to open a directory folder under windows OS, just like you right click one of your local folder and select open. 
Any suggestions?
You may simply try this:
os.startfile(whatever_valid_filename)
This starts the default OS application for whatever_valid_filename, meaning Explorer for a folder name, default notepad for a .txt file, etc.
PyQt - How to open a directory folder?