from PySide2 import QtWidgets, QtGui, QtCore
class Example(QtWidgets.QMainWindow):  
    def __init__(self):
        super(Example, self).__init__()
        self.initUI()
        
    def initUI(self):
        btn = QtWidgets.QToolButton()
        bar = self.addToolBar('toolbar')
        bar.addWidget(btn)
        btn.setText('btn')
        btn.setFixedSize(32, 32)

        menu = QtWidgets.QMenu()
        action = menu.addAction('hello')
        btn.setMenu(menu)
        self.show()
Example = Example()