あえてpos_hintを使わずに中央揃えを試みる

part1(fail).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder

runTouchApp(Builder.load_string('''
FloatLayout:
    Button:
        text: 'Kivy'
        center: root.center
'''))

part2(success).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder

runTouchApp(Builder.load_string('''
FloatLayout:
    Button:
        text: 'Kivy'
        size_hint: None, None
        center: root.center
'''))

part3(fail).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder

runTouchApp(Builder.load_string('''
FloatLayout:
    Button:
        text: 'Kivy'
        size_hint: None, None
        size: root.size
        center: root.center
'''))

part3_without_kv(fail).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.factory import Factory


root = Factory.FloatLayout()
button = Factory.Button(text='Kivy', size_hint=(None, None, ))
root.add_widget(button)

root.bind(size=button.setter('size'), center=button.setter('center'))

runTouchApp(root)

part4(fail).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder

runTouchApp(Builder.load_string('''
Widget:
    Button:
        text: 'Kivy'
        size_hint: None, None
        size: root.size
        center: root.center
'''))

part5(success).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder


root = Builder.load_string('''
FloatLayout:
    Button:
        text: 'Kivy'
        x: root.center_x - self.width / 2
        y: root.center_y - self.height / 2
        size_hint: .5, .5
''')


runTouchApp(root)

part6(success).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder


root = Builder.load_string('''
FloatLayout:
    Button:
        text: 'Kivy'
        right: root.center_x + self.width / 2
        top: root.center_y + self.height / 2
        size_hint: .5, .5
''')


runTouchApp(root)

part7(success).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder


root = Builder.load_string('''
FloatLayout:
    Button:
        text: 'Kivy'
        center_x: root.center_x + self.width * 0
        center_y: root.center_y + self.height * 0
        size_hint: .5, .5
''')


runTouchApp(root)

part8(success).py

from kivy.config import Config
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 300)
from kivy.app import runTouchApp
from kivy.lang import Builder


root = Builder.load_string('''
FloatLayout:
    Button:
        text: 'Kivy'
        center: root.center or self.size
        size_hint: .5, .5
''')


runTouchApp(root)