Slideshow error

c.py

#To use it you need to put pictures into /pictures/

from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition, SwapTransition,WipeTransition
from kivy.uix.image import AsyncImage
from kivy.uix.boxlayout import BoxLayout
import kivy
from kivy.uix.widget import Widget
from kivy.app import App
import os, glob
from kivy.lang import Builder
from kivy.uix.label import Label
import time
from threading import Timer
from random import random

kvs='''
BoxLayout:
	orientation: 'vertical'
	BoxLayout:
		id: globebox
		ScreenManager:
			id: sm1
			Screen:
				name: '1'
				BoxLayout:
					id: box1
			Screen:
				name: '2'
				BoxLayout:
					id: box2
			Screen:
				name: '3'
				BoxLayout:
					id: box3
<FullImage>:
    canvas:
        Color:
            rgb: (1, 1, 1)
        Rectangle:
            texture: self.texture
            size: self.size
            pos: self.pos					
					'''
root = Builder.load_string(kvs)	
class FullImage(AsyncImage):
    pass
filelist = glob.glob('pictures/*')
print filelist
def picpic():
	print filelist
	for num, pic in enumerate(filelist):
		ran = random()*len(filelist)
		if num+1 > ran: return pic
root.ids.sm1.transition=FadeTransition()
def chsc():
	
	aaa= picpic()
	print 'returns', aaa
	aaa=FullImage(source = str(aaa))
	print 'screen', root.ids.sm1.current
	if root.ids.sm1.current == '1':
		root.ids.box2.add_widget(aaa)
		root.ids.sm1.current = '2'
		root.ids.box3.clear_widgets()
		print 'change to sc2'

	elif root.ids.sm1.current == '2':
		root.ids.box3.add_widget(aaa)
		root.ids.sm1.current = '3'
		root.ids.box1.clear_widgets()
		print 'change to sc1'
	elif root.ids.sm1.current == '3':
		root.ids.box1.add_widget(aaa)
		root.ids.sm1.current = '1'
		root.ids.box2.clear_widgets()
		print 'change to sc2'
	print 'screen', root.ids.sm1.current
	Timer(1, timing).start()
def timing():
	if root.ids.sm1.current_screen.transition_progress> 0.90:
		chsc()

	

class MainApp(App): #Main function#
	def build(self):
		chsc()
		return root
if __name__ == '__main__':
    MainApp().run()