kivy - clear screen after saving image? - python-2.7

Question

Using kivy 1.9.1/2
What I would like to achieve :
Saving the image when clicking 'OK' then clearing the screen. So the next time I enter the Signature screen, the screen doesn't contain anything but the 'OK' button.
My Issue:
canvas.clear doesn't work. I probably integrated it the wrong way.
Can someone help ?
I'm a beginner and have been stuck on this for days.
Thanks in advance.
#*** PYTHON FILE
from kivy.app import App
from kivy.lang import Builder
from kivy.config import Config
from kivy.uix.screenmanager import ScreenManager, Screen, SlideTransition
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.graphics import Line
import datetime
Config.set('graphics', 'width', '1024')
Config.set('graphics', 'height', '768')
class Other(Screen):
pass
class Painter(Widget):
def __init__(self,**kwargs):
super(Painter, self).__init__(**kwargs)
self.canvas.clear()
def on_touch_down(self, touch):
with self.canvas:
touch.ud['line'] = Line(points=(touch.x, touch.y))
def on_touch_move(self, touch):
touch.ud['line'].points += [touch.x, touch.y]
class Signature(Screen):
def Save_image(self):
from PIL import Image, ImageFont, ImageDraw
self.export_to_png("test.png")
img=Image.open("test.png")
draw=ImageDraw.Draw(img)
font=ImageFont.truetype("/usr/share/fonts/truetype/noto/NotoSans-Bold.ttf",24, encoding="unic")
draw.text((0,0),str("{:%d %b %y-%Hh%Mm%Ss}".format(datetime.datetime.now())), (255,255,255), font=font)
img.save("test.png")
class ScreenManagement(ScreenManager):
pass
presentation = Builder.load_file("appdessin.kv")
class MainApp(App):
def build(self):
return presentation
if __name__ == '__main__':
MainApp().run()
#***KV FILE
#:import datetime datetime
#:kivy 1.9.1
ScreenManagement:
Other:
Signature:
<Other>:
name: "other"
id: otherScreen
BoxLayout:
Button:
text: "OK"
on_release:
root.manager.transition.direction = 'right'
app.root.current = "signature"
<Signature>:
name: "signature"
id: signatureScreen
BoxLayout:
Painter
Button:
background_color: 0,0,1,1
font_size: 32
size_hint: (0.1, 0.1)
text: "OK"
pos_hint: {"right":1, "bottom":1}
on_release:
root.Save_image()
root.manager.transition.direction = 'right'
app.root.current = "other"

Answer 1

As suggested by niavlys on kivy's google group, the answer is to clear the canvas everytime it is loaded. Obvious, but to achieve this, one has to declare in the kv file, in the screen:
on_pre_enter:
sign.canvas.clear()
BoxLayout:
Painter
id: sign