Untitled

from PySide2.QtWidgets import QApplication, QMainWindow, QWidget, QLabel, QVBoxLayout
from sys import argv, exit
class Win(QMainWindow):
    def __init__(self):
        super().__init__()
        self.ui()
        self.show()

    def ui(self):
        main_widget = QWidget()
        main_layout = QVBoxLayout()
        label = QLabel("A")
        main_layout.addWidget(label)
        main_widget.setLayout(main_layout)
        self.setCentralWidget(main_widget)
        self.grabMouse()

if __name__ == '__main__':
    app = QApplication(argv)
    window = Win()
    exit(app.exec_())