title: Why mouseMoveEvent does nothing in PyQt5
body: <p>I try to use mouseMoveEvent and mousePressEvent in PyQt5 and Python3.5, but there is nothing when I click my mouse. My code is as following, is there something wrong?</p>
  
  <pre><code>from PyQt5 import QtWidgets, QtGui, QtCore
  
  class Window(QtWidgets.QMainWindow):
      def __init__(self):
          QtWidgets.QMainWindow.__init__(self)
          widget = QtWidgets.QWidget(self)
          layout = QtWidgets.QVBoxLayout(widget)
          self.graphicsView = QtWidgets.QGraphicsView()
          self.graphicsView.setCursor(QtCore.Qt.CrossCursor)
          self.graphicsView.setObjectName("graphicsView")
          layout.addWidget(self.graphicsView)
          self.setCentralWidget(widget)
  
      def mouseMoveEvent(self, event):
          if event.buttons() == QtCore.Qt.NoButton:
              print("Simple mouse motion")
          elif event.buttons() == QtCore.Qt.LeftButton:
              print("Left click drag")
          elif event.buttons() == QtCore.Qt.RightButton:
              print("Right click drag")
  
      def mousePressEvent(self, event):
          if event.button() == QtCore.Qt.LeftButton:
              print("Press!")
  
  if __name__ == '__main__':
      import sys
      app = QtWidgets.QApplication(sys.argv)
      win = Window()
      win.show()
      sys.exit(app.exec_())
  </code></pre>