import sys
from PyQt5 import QtWidgets
from PyQt5.QtGui import QCursor


class MyTextEdit(QtWidgets.QTextEdit):

    def __init__(self,parent=None):
        super(MyTextEdit,self).__init__(parent)

    def focusInEvent(self,event):
        super(MyTextEdit,self).focusInEvent(event)
        QtWidgets.QToolTip.showText(QCursor.pos(), 'tooltip')

class MainFrame(QtWidgets.QWidget):

    def __init__(self):
        super(MainFrame,self).__init__()
        self.initUI()

    def initUI(self):
        self.te=MyTextEdit()
        hlayout=QtWidgets.QHBoxLayout()
        hlayout.addWidget(self.te)
        hlayout.addStretch()
        self.setLayout(hlayout)
        self.show()


class MainWindow(QtWidgets.QMainWindow):

    def __init__(self):
        super(MainWindow,self).__init__()

        self.main_frame=MainFrame()
        self.setCentralWidget(self.main_frame)
        self.setGeometry(100,100,800,600)
        self.show()


if __name__=='__main__':

    app=QtWidgets.QApplication(sys.argv)
    mainwindow=MainWindow()
    sys.exit(app.exec_())