qt_foo.py

#!/usr/bin/env python3

import sys

from PyQt5.QtCore import pyqtSlot
from PyQt5.QtWidgets import QApplication, QDialog
from PyQt5.uic import loadUi

class FooWindow(QDialog):

    def __init__(self):
        super(FooWindow, self).__init__()
        loadUi('foo.ui', self)

    @pyqtSlot()
    def on_pushButton_clicked(self):
        print("Hello World")


if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = FooWindow()
    window.show()
    sys.exit(app.exec_())