import sys
import serial.tools.list_ports
from PyQt5 import QtWidgets, uic


qtcreator_file  = "D:/Programy/Qt/projects/mainwindowPWSW.ui"
Ui_MainWindow, QtBaseClass = uic.loadUiType(qtcreator_file)


class MyWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self):
        QtWidgets.QMainWindow.__init__(self)
        Ui_MainWindow.__init__(self)
        self.setupUi(self)
        self.detectButton.clicked.connect(self.detectPort)

    def detectPort(self):
        for port in serial.tools.list_ports.comports():
            self.portComboBox.addItem(port.device)



if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    window = MyWindow()
    window.show()
    sys.exit(app.exec_())