Original Post can be find in the link below, I paste here in case it can not be access in the future.
Quote from JoDaRober
If you have issues with the dialog not appearing at the center of the screen, you can center it when showing the UI. I prefer to override QDialog’s show method for this, since it will have to happen after any widget creation that can change the dimensions of the dialog, and that include’s QtDialog’s show() method. Here’s an example: