pyqt/basic/windows_1.py

import sys
from random import randint
from PyQt5.QtWidgets import QApplication, QMainWindow, QVBoxLayout, QPushButton, QWidget, QInputDialog, QLineEdit, QLabel


class AnotherWindow(QWidget):
    """
    This "window" is a Qwidget. If it has no parent, it 
    will appear as a free-floating window.
    """

    def __init__(self):
        super().__init__()
        layout = QVBoxLayout()
        self.label = QLabel("Another Window %d" % randint(0, 100))
        layout.addWidget(self.label)
        self.setLayout(layout)


class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()
        self.w = None  # No external window yet

        self.setWindowTitle("My App")

        layout = QVBoxLayout()

        self.button = QPushButton("Press me for a dialog!")
        self.button.clicked.connect(self.show_new_window)
        layout.addWidget(self.button)

        container = QWidget()
        container.setLayout(layout)

        self.setCentralWidget(container)

    def show_new_window(self, is_checked):
        print("Button clicked!", is_checked)
        if self.w is None:
            self.w = AnotherWindow()
            self.w.show()
        else:
            self.w.close()
            self.w = None  # Discard reference, close window.


app = QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec_()