Kivy Scrollview in kv language - id not defined - scrollview

Question

Trying to figure out how to implement a straight-forward ScrollView in KV language based on the examples given in the documentation. I cannot believe I cannot find a single example of this (only parts of solution), so I thought it would be easy. Turns out it's not.
My issue is that I need to populate my scrollable grid layout with a list of labels from within the kivy script, using add_widget. That's because I have a variable number of of labels to add (although that number is fixed in the below example to make things simple). However the program wouldn't let me do it, saying that the ID I defined for the grid layout object is not defined. Therefore I am not able to add the labels to the grid layout.
NameError: name '_gridlayout' is not defined
Any help appreciated. Thanks
from kivy.app import App
from kivy.uix.scrollview import ScrollView
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.lang import Builder
from kivy.uix.floatlayout import FloatLayout
Builder.load_string('''
<MainScreen>:
AnchorLayout:
anchor_x: 'center'
anchor_y: 'top'
size_hint: 1, .2
Label:
text: "Random text"
AnchorLayout:
anchor_x: 'center'
anchor_y: 'bottom'
size_hint: 1, .8
ScrollView:
GridLayout:
id: _gridlayout
cols: 1
padding: 10
spacing: 10
size_hint_y: None
width: 500
''')
class MainScreen(FloatLayout):
def __init__(self, **kwargs):
self.buildList()
super(MainScreen, self).__init__(**kwargs)
def buildList(self):
for i in range(30):
btn = Label(text=str(i), size_hint_y=None, height=40)
_gridlayout.add_widget(btn) # <- ERROR
class SMApp(App):
def build(self):
return MainScreen()
if __name__ == '__main__':
SMApp().run()
UPDATE: Corrected script below.
from kivy.app import App
from kivy.uix.scrollview import ScrollView
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.lang import Builder
from kivy.uix.floatlayout import FloatLayout
Builder.load_string('''
<MainScreen>:
AnchorLayout:
anchor_x: 'center'
anchor_y: 'top'
size_hint: 1, .2
Label:
text: "Random text"
AnchorLayout:
anchor_x: 'center'
anchor_y: 'bottom'
size_hint: 1, .8
ScrollView:
GridLayout:
id: _gridlayout
cols: 1
padding: 10
spacing: 10
size_hint_y: None
width: 500
''')
class MainScreen(FloatLayout):
def __init__(self, **kwargs):
super(MainScreen, self).__init__(**kwargs)
self.buildList()
def buildList(self):
for i in range(30):
btn = Label(text=str(i), size_hint_y=None, height=40)
self.ids._gridlayout.add_widget(btn)
self.ids._gridlayout.bind(minimum_height=self.ids._gridlayout.setter('height'))
class SMApp(App):
def build(self):
return MainScreen()
if __name__ == '__main__':
SMApp().run()

Answer 1

Kivy is very picky about calling Widget.__init__() first. So if you overwrite __init__ of a widget be sure to first call super().__init__, otherwise you can get errors like you encountered or "random" crashes.
To fix change
class MainScreen(FloatLayout):
def __init__(self, **kwargs):
self.buildList()
super(MainScreen, self).__init__(**kwargs)
to
class MainScreen(FloatLayout):
def __init__(self, **kwargs):
super(MainScreen, self).__init__(**kwargs)
self.buildList()