How to open an external Python file as a subwindow of the main application?

Question

I’m a beginner in Python and I’m developing an MDI application. Knowing that it will be extensive, I thought it would be better to separate the code into files. I already have two:

main py. (main application with mdiArea) users py. (form that browses the records of the users table)

I can spin the users py. from the main py. using the import process and then calling: subprocess.call(["python", "users.py"])

I need the Users form to open as a subwindow from the main window. Could someone please give a hint? Thanks!