Think Python - Denison University

190 Chapter 19. Casestudy: Tkinterdef __init__(self, item):self.canvas = item.canvasself.tag = item.tagself.bind('', self.select)self.bind('', self.drag)self.bind('', self.drop)The init method takes an Item as a parameter. It copies the attributes of the Item and then createsbindings for threeevents: abutton press,button motion, and button release.The event handler select stores the coordinates of the current event and the original color of theitem, then changes thecolor toyellow:def select(self, event):self.dragx = event.xself.dragy = event.yself.fill = self.cget('fill')self.config(fill='yellow')cgetstandsfor“getconfiguration;”ittakesthenameofanoptionasastringandreturnsthecurrentvalue of that option.drag computes how far the object has moved relative to the starting place, updates the stored coordinates,and then moves the item.def drag(self, event):dx = event.x - self.dragxdy = event.y - self.dragyself.dragx = event.xself.dragy = event.yself.move(dx, dy)This computation isdone inpixel coordinates; thereisno need toconvert toCanvas coordinates.Finally,droprestorestheoriginal color of the item:def drop(self, event):self.config(fill=self.fill)You can use theDraggable class to add drag-and-drop capability to an existing item. For example,here is a modified version of make_circle that uses circle to create an Item and Draggable tomake it draggable:def make_circle(event):pos = ca.canvas_coords([event.x, event.y])item = ca.circle(pos, 5, fill='red')item = Draggable(item)This example demonstrates one of the benefits of inheritance: you can modify the capabilities ofa parent class without modifying its definition. This is particularly useful if you want to changebehavior defined inamodule you did not write.