Return value based on event in python -

i need value calculated calculate function being called once event being fired on_release function. value outside class.

class annotate(object):     def __init__(self,diff):         self.differnce = diff         self.x0 = none         self.y0 = none         self.x1 = none         self.y1 = none         self.ax = plt.gca()         self.ax.figure.canvas.mpl_connect('button_press_event', self.on_press)         self.ax.figure.canvas.mpl_connect('button_release_event', self.on_release)      def on_press(self, event):         print 'press'         self.x0 = event.xdata         self.y0 = event.ydata         # return x0,y0      def on_release(self, event):         print 'release'         x1 = event.xdata         y1 = event.ydata         self.calculate(int(self.x0), int(self.y0), int(x1), int(y1))      def calculate(self, a, b, c, d):         print a, b, c, d         print self.differnce         sum = 0         count = 0         in range(a, c):             j in range(b, d):                 sum = sum + self.differnce[i,j]                 count = count+1         print sum         print count         if (sum==0):             if(count==0):                 print "try again"         else:             return sum/count  = annotate(diff) print