QTreeWidget的topLevelItemCount属性是一个只读属性，用于保存树型部件中顶层项的个数，可以通过topLevelItemCount（）方法获取属性值，缺省值为0。
当树型部件中顶层项的数目变化时，topLevelItemCount自动跟随变化。
案例：
def initTopItems(self):
cdriver = QtWidgets.QTreeWidgetItem(1,2,["c:\","NTFS","10G"])
ddriver = QtWidgets.QTreeWidgetItem(["d:\","NTFS","20G"])
edriver = QtWidgets.QTreeWidgetItem(["e:\","NTFS","30G"])
print("1.self.treeWidget.topcount=", self.treeWidget.topLevelItemCount())
self.treeWidget.addTopLevelItem(cdriver)
print( "2.self.treeWidget.topcount=",self.treeWidget.topLevelItemCount())
self.treeWidget.addTopLevelItem(ddriver)
print("3.self.treeWidget.topcount=", self.treeWidget.topLevelItemCount())
self.treeWidget.addTopLevelItem(edriver)
print("4.self.treeWidget.topcount=", self.treeWidget.topLevelItemCount())
案例输出：
1.self.treeWidget.topcount= 0
2.self.treeWidget.topcount= 1
3.self.treeWidget.topcount= 2
4.self.treeWidget.topcount= 3
