from PyQt6.QtCore import QSize
from PyQt6.QtWidgets import QApplication, QMainWindow, QPushButton

import sys

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("Hello World")
        button = QPushButton("Click Me!")

        self.setMinimumSize(QSize(200, 100))
        self.setMaximumSize(QSize(900, 800))

        self.setCentralWidget(button)

app = QApplication(sys.argv)

window = MainWindow()
window.show() # windows are hidden by default

# start event loop
app.exec()