# MyComboBox.py
from PyQt5.QtWidgets import QComboBox
from PyQt5.QtCore import pyqtSignal       
class MyComboBox(QComboBox):
  clicked = pyqtSignal()     #创建一个信号
  def showPopup(self):      #重写showPopup函数
    self.clicked.emit()     #发送信号
    super(MyComboBox, self).showPopup()   # 调用父类的showPopup()

# test_ui.py
    self.PrintersList = MyComboBox(self.groupBox) # 修改后
    # self.PrintersList = QtWidgets.QComboBox(self.groupBox) # 修改前

# main_loop.py
  self.PrintersList.clicked.connect(self.scan_printer_list_slot)    # 信号与槽函数的绑定
 # 槽函数的实现
  def scan_printer_list_slot(self):
   print("扫描打印机并刷新列表")

from PyQt5 import QtCore, QtGui, QtWidgets
import sys
 
class CheckableComboBox(QtWidgets.QComboBox):
  def __init__(self, parent=None):
    super(CheckableComboBox, self).__init__(parent)
    self.setModel(QtGui.QStandardItemModel(self))
    self.view().pressed.connect(self.handleItemPressed)
    self.checkedItems = []
    self.view().pressed.connect(self.get_all)
    self.view().pressed.connect(self.getCheckItem)
    self.status = 0
 
  def handleItemPressed(self, index):              #这个函数是每次选择项目时判断状态时自动调用的，不用管（自动调用）
    item = self.model().itemFromIndex(index)
    if item.checkState() == QtCore.Qt.Checked:
      item.setCheckState(QtCore.Qt.Unchecked)
    else:
      item.setCheckState(QtCore.Qt.Checked)
 
  def getCheckItem(self):
    # getCheckItem方法可以获得选择的项目列表，自动调用。
    for index in range(1,self.count()):
      item = self.model().item(index)
      if item.checkState() == QtCore.Qt.Checked:
        if item.text() not in self.checkedItems:
          self.checkedItems.append(item.text())
      else:
        if item.text() in self.checkedItems:
          self.checkedItems.remove(item.text())
    print("self.checkedItems为：",self.checkedItems)
    return self.checkedItems          #实例化的时候直接调用这个self.checkedItems就能获取到选中的值，不需要调用这个方法，方法会在选择选项的时候自动被调用。
 
  def get_all(self):              #实现全选功能的函数（自动调用）
    all_item = self.model().item(0)
 
    for index in range(1,self.count()):    #判断是否是全选的状态，如果不是，全选按钮应该处于未选中的状态
      if self.status ==1:
        if self.model().item(index).checkState() == QtCore.Qt.Unchecked:
          all_item.setCheckState(QtCore.Qt.Unchecked)
          self.status = 0
          break
 
    if all_item.checkState() == QtCore.Qt.Checked:
      if self.status == 0 :
        for index in range(self.count()):
          self.model().item(index).setCheckState(QtCore.Qt.Checked)
          self.status = 1
 
    elif all_item.checkState() == QtCore.Qt.Unchecked:
      for index in range(self.count()):
        if self.status == 1 :
          self.model().item(index).setCheckState(QtCore.Qt.Unchecked)
      self.status = 0
 
if __name__ == "__main__":
  app = QtWidgets.QApplication(sys.argv)
  dialog = QtWidgets.QMainWindow()
  mainWidget = QtWidgets.QWidget()
  dialog.setCentralWidget(mainWidget)
  ComboBox = CheckableComboBox(mainWidget)
  ComboBox.addItem("全选")
  for i in range(6):
    ComboBox.addItem("Combobox Item " + str(i))
  dialog.show()
  sys.exit(app.exec_())

ComboBox.activated.connect(self.get_checkedItems_slot) #推荐
ComboBox.highlighted.connect(self.get_checkedItems_slot)
ComboBox.currentIndexChanged.connect(self.get_checkedItems_slot)