import sys
from PyQt5.QtWidgets import QApplication, QWidget
from PyQt5.QtCore import QPointF ,QRectF ,Qt
from PyQt5.QtGui import QPainterPath ,QPainter ,QPen

class App(QWidget):

    def __init__(self):
        super().__init__()
        self.setWindowTitle('hogetta')
        self.setGeometry( 500, 50 , 500 , 500 )
        self.setMouseTracking (True)  # <----------------*****
        self.plist=[]
        self.plist.append( QPointF(10,250) )  # start point
        self.plist.append( QPointF(100,100) )  #
        self.plist.append( QPointF(300,200) )  #
        self.plist.append( QPointF(490,250) )  #
        self.a = QPointF(4,4) # Tolerance in detecting point
        self.pselected = -1             # selectedpoint
        self.pmove = 0                  # move flag
        self.show()

    def mouseMoveEvent(self ,e):
        if self.pmove :  # Dorag ing
            self.plist[self.pselected] = e.pos()
            self.update()
        else:            # non Dorag ing
            self.pselected = -1
            rc = QRectF ( e.pos() - self.a , e.pos() + self.a )
            for i, l in  enumerate( self.plist ) :
                if rc.contains(  l ) :
                    self.pselected = i
                    # self.setWindowTitle('HIT  {}'.format(str(i)))
                    self.update()
                    break
    def mousePressEvent(self ,e):
        if self.pselected != -1 :
            self.pmove = True
    def mouseReleaseEvent(self ,e):
        self.pmove = False
        self.pselected = -1

    def paintEvent (self,e):
        # 与えられたパラメータでパスを設定した後でパスをドローする
        path = QPainterPath( self.plist[0] )
        path.cubicTo( self.plist[1] , self.plist[2] , self.plist[3] )

        painter= QPainter()
        painter.begin(self) 
        painter.setPen( QPen( Qt.black , 2, style=Qt.SolidLine ) )
        painter.drawPath(path)

        painter.setPen( QPen( Qt.red , 2, style=Qt.DotLine ) )
        painter.drawLine( self.plist[0] , self.plist[1] ) # sub
        painter.drawLine( self.plist[2] , self.plist[3] ) # sub

        if self.pselected != -1:
            painter.drawRect( QRectF(QPointF(self.plist[self.pselected]-self.a) ,
                QPointF(self.plist[self.pselected]+self.a) ))

        painter.end() 

if __name__ == "__main__":
    app = QApplication(sys.argv)
    ex = App()
    sys.exit(app.exec_())