Difference between using kv file and python class for initializing a widget

Question

I was learning how to create an App with Kivy and I see that the App looks different when I use Python vs kv language.

What I am trying to do is to get the background colored red. When I do it with Python, I get a little red box at the bottom left corner (probably, the default 100px kivy widget size). When I do it with the kv file, I get the entire background colored red

Here is my python code -

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import *
class MyWidget(Widget):
 def __init__(self, **kwargs):
 super(MyWidget, self).__init__(**kwargs)
 with self.canvas:
 Color(1,0,0,1)
 Rectangle(size = self.size, pos = self.pos)
class MyWidgetApp(App):
 def build(self):
 x = MyWidget()
 return x
 pass
if __name__ == "__main__":
 MyWidgetApp().run()

Here is my kv file

<MyWidget@Widget>:
 canvas:
 Color:
 rgba: 1, 0, 0, 1.0
 Rectangle:
 size: self.size
 pos: self.pos

Why does this happen? And how can I set the background color from Python? I am using Python 2.7 on a Linux machine

Answer 1

The problem is that kv automatically makes a binding to update the red rectangle position and size, while python doesn't (and can't). You have to use the bind method to create your own binding. I have a blog post about this here - coincidentally, it addresses precisely your example.