from PyQt5.Qt import *

if __name__ == '__main__':
    app = QApplication([])

    x = QTreeWidget()
    x.setContextMenuPolicy(Qt.CustomContextMenu)
    i1 = QTreeWidgetItem(x, ["A"])
    i2 = QTreeWidgetItem(i1, ["bar", "i", "ii"])
    i3 = QTreeWidgetItem(i2, ["baz", "a", "b"])

    def callback(point):
        menu = QMenu()
        menu.addAction("Foo")
        menu.addSeparator()
        menu.addAction("Bar")
        menu.exec_(x.mapToGlobal(point))

    x.customContextMenuRequested.connect(callback)
    x.show()
    app.exec_()