How to add a variable in Directory MSI table using msilib

I want to use a path edit control in my setup application I am designing using Python msilib. But I am getting an error in the MSI saying:

The specified path is unavailable.

Can somebody help me find the reason 开发者_开发技巧for this? I think I haven't made any entry in the Directory table which can cause the error. How can I do it using msilib?

---

This is MSI error: 1314 http://msdn.microsoft.com/en-us/library/aa372835(v=vs.85).aspx

This can happen if the path that you've specified is invalid (perhaps you have a space in the name like C:\Program Files and should enclose it in quotes like "C:\Program Files") or this error can also happen if you are specifying a path that already exists.