本文是对《Python Qt GUI快速编程》的第10章的例子events用Python3+PyQt5进行改写，涉及到重新实现QWidget的事件处理程序。本例子涉及到上下文菜单，鼠标事件，键盘事件，可作为重新实现事件处理程序的参考。

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注：在创建上下文菜单最简单的方式使用Qwidget.addAction()把动作添加到窗口部件中，再把窗口部件的上下文菜单策略设置为Qt.ActionsContextMenu即可，但是如果像本例子一样要根据不同的状态来提供不同的选项，则要重新实现上下文菜单事件处理程序。

#!/usr/bin/env python3
import sys
from PyQt5.QtCore import (QEvent, QTimer, Qt)
from PyQt5.QtWidgets import (QApplication, QMenu, QWidget)
from PyQt5.QtGui import QPainter

class Widget(QWidget):

  def __init__(self, parent=None):
    super(Widget, self).__init__(parent)
    self.justDoubleClicked = False
    self.key = ""
    self.text = ""
    self.message = ""
    self.resize(400, 300)
    self.move(100, 100)
    self.setWindowTitle("Events")
    QTimer.singleShot(0, self.giveHelp) # Avoids first resize msg


  def giveHelp(self):
    self.text = "Click to toggle mouse tracking"
    self.update()


  def closeEvent(self, event):
    print("Closed")


  def contextMenuEvent(self, event):
    menu = QMenu(self)
    oneAction = menu.addAction("&One")
    twoAction = menu.addAction("&Two")
    oneAction.triggered.connect(self.one)
    twoAction.triggered.connect(self.two)
    if not self.message:
      menu.addSeparator()
      threeAction = menu.addAction("Thre&e")
      threeAction.triggered.connect(self.three)
    menu.exec_(event.globalPos())


  def one(self):
    self.message = "Menu option One"
    self.update()


  def two(self):
    self.message = "Menu option Two"
    self.update()


  def three(self):
    self.message = "Menu option Three"
    self.update()


  def paintEvent(self, event):
    text = self.text
    i = text.find("\n\n")
    if i >= 0:
      text = text[0:i]
    if self.key:
      text += "\n\nYou pressed: {0}".format(self.key)
    painter = QPainter(self)
    painter.setRenderHint(QPainter.TextAntialiasing)
    painter.drawText(self.rect(), Qt.AlignCenter, text)
    if self.message:
      painter.drawText(self.rect(), Qt.AlignBottom|Qt.AlignHCenter,
               self.message)
      QTimer.singleShot(5000, self.clearMessage)
      QTimer.singleShot(5000, self.update)

  def clearMessage(self):
    self.message=""

  def resizeEvent(self, event):
    self.text = "Resized to QSize({0}, {1})".format(
              event.size().width(), event.size().height())
    self.update()


  def mouseReleaseEvent(self, event):
    if self.justDoubleClicked:
      self.justDoubleClicked = False
    else:
      self.setMouseTracking(not self.hasMouseTracking())
      if self.hasMouseTracking():
        self.text = "Mouse tracking is on.\n"+\
            "Try moving the mouse!\n"+\
            "Single click to switch it off"
      else:
        self.text = "Mouse tracking is off.\n"+\
                      "Single click to switch it on"
      self.update()


  def mouseMoveEvent(self, event):
    if not self.justDoubleClicked:
      globalPos = self.mapToGlobal(event.pos())
      self.text = "The mouse is at\nQPoint({0}, {1}) "+\
          "in widget coords, and\n"+\
          "QPoint({2}, {3}) in screen coords".format(
          event.pos().x(), event.pos().y(), globalPos.x(),
          globalPos.y())
      self.update()


  def mouseDoubleClickEvent(self, event):
    self.justDoubleClicked = True
    self.text = "Double-clicked."
    self.update()


  def keyPressEvent(self, event):
    self.key = ""
    if event.key() == Qt.Key_Home:
      self.key = "Home"
    elif event.key() == Qt.Key_End:
      self.key = "End"
    elif event.key() == Qt.Key_PageUp:
      if event.modifiers() & Qt.ControlModifier:
        self.key = "Ctrl+PageUp"
      else:
        self.key = "PageUp"
    elif event.key() == Qt.Key_PageDown:
      if event.modifiers() & Qt.ControlModifier:
        self.key = "Ctrl+PageDown"
      else:
        self.key = "PageDown"
    elif Qt.Key_A <= event.key() <= Qt.Key_Z:
      if event.modifiers() & Qt.ShiftModifier:
        self.key = "Shift+"
      self.key += event.text()
    if self.key:
      self.key = self.key
      self.update()
    else:
      QWidget.keyPressEvent(self, event)


  def event(self, event):
    if (event.type() == QEvent.KeyPress and
      event.key() == Qt.Key_Tab):
      self.key = "Tab captured in event()"
      self.update()
      return True
    return QWidget.event(self, event)

if __name__ == "__main__":
  app = QApplication(sys.argv)
  form = Widget()
  form.show()
  app.exec_()

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