[PyQt] Can't neither setWindowFlags nor setFixedSize of a subwindow.

[JokerSob]

Hi, guys. My MDI application used to have all the code in one single file and everything worked, but now that I split the code I can't set the subwindow flags and its fixed size. No error messages. The subwindow opens just as if the code was no there. Everything else works exactly that way I want. The Mdi is in main.py and the subwindow in usuarios.py. Any ideas? Here's the code for both files. Thanks.

import usuarios

line 3, in <module>
import usuarios
ModuleNotFoundError: No module named 'usuarios'