# -*- coding: utf-8 -*-
from PyQt4 import QtGui, QtCore
import sys

class MyWindow(QtGui.QWidget):
    def __init__(self, file, parent=None):
        QtGui.QWidget.__init__(self,parent)
        self.spisok= ['a','b','c']
        self.btn = QtGui.QPushButton('try!',self)
        self.btn.setGeometry(25,25,25,25)
        self.btn.clicked.connect(self.onClickTry)
        self.i = 0
    def onClickTry(self):
        try:
            if self.i <=len(self.spisok):
                print(self.spisok[self.i])
                self.i+=1
        except:
            print('stop!')

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    window = MyWindow(sys.argv)
    window.resize(400,240)
    window.show()
    sys.exit(app.exec_())