new_CustomMainWindowClass.setWindowFlags(QtCore.Qt.Widget)

def setWidgetMode(self):
    self.setWindowFlags(QtCore.Qt.Widget)
    self.menuBar().hide()
    self.statusBar().hide()
    self.uiList['main_layout'].setContentsMargins(0, 0, 0, 0)
def setWidgetMode(self, win_ui):
    win_ui.setWindowFlags(QtCore.Qt.Widget)
    win_ui.menuBar().hide()
    win_ui.statusBar().hide()
    win_ui.uiList['main_layout'].setContentsMargins(0, 0, 0, 0)