Kivy using the StackLayout - kivy

Question

I want to use a StackLayout but it only display one button and not two as I expected. The code is:
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.lang import Builder
from kivy.uix.textinput import TextInput
from kivy.uix.stacklayout import StackLayout
from kivy.uix.button import Button
Builder.load_string("""
<ScreenUI>:
orientation: 'lr-bt'
Button:
text: 'Button 1'
Button:
text: 'Button 2'
""")
class ScreenUI(StackLayout):
pass
class WidgetApp(App):
def build(self):
app = ScreenUI()
return app
if __name__ == '__main__':
WidgetApp().run()
How do I use a StackLayout and add a List of Button?
Update (to includes comments): To obtain the resize behaviour I used
Builder.load_string("""
<ScreenUI>:
input1: input1
button1: button1
height: self.input1.height
spacing: 5
orientation: 'horizontal'
id: layout1
Label:
text: 'Button 1'
id: button1
size: len(self.text) * root.input1.font_size, 2 * root.input1.font_size
size_hint: None, None
TextInput:
id: input1
text: 'Button 2'
size: root.width - root.button1.width - root.spacing, 2 * self.font_size
size_hint: None, None
""")
Does a more elegant method exists?

Answer 1

It looks like this is a size_hint problem. The buttons have no size specified, so the StackLayout automatically sizes them with their size hints. However, its layout doesn't depend on any sensible way of doing that, so it just makes them 100% of its own size...meaning only one fits in the window. That's the one you see.
Here's a slightly modified version of your example where sizes are specified. It works fine for me, with two differently sized buttons placed in the bottom left:
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.lang import Builder
from kivy.uix.textinput import TextInput
from kivy.uix.stacklayout import StackLayout
from kivy.uix.button import Button
Builder.load_string("""
<ScreenUI>:
orientation: 'lr-bt'
Button:
text: 'Button 1'
size: 100, 100
size_hint: None, None
Button:
text: 'Button 2'
size_hint: None, None
size: 200, 100""")
class ScreenUI(StackLayout):
pass
class WidgetApp(App):
def build(self):
app = ScreenUI()
return app
if __name__ == '__main__':
WidgetApp().run()
In general, the way to fix this is to give them a sensible size hint for whatever your particular goal is, or to set their sizes manually as I did. If you want a certain number to be sized properly to fill a space or similar, a GridLayout or BoxLayout may be more suitable.