How to pass data to a custom widget from kv language - python-3.x

Question

I want to pass a variable to the class of a custom widget to construct it.
Python:
class Customwidget(BoxLayout)
mystring = StringProperty()
def __init__(self, **kwargs):
super(Customwidget, self).__init__(**kwargs)
print(mystring)
Kivy
Root:
BoxLayout:
orientation: 'vertical'
Customwidget:
mystring: 'stringcontents'
When this is executed, mystring remains empty. How do I pass it to the class such that it can be used within init for logic in defining the widget's contents?

Answer 1

Problem - empty string
When this is executed, mystring remains empty. How do I pass it to the
class such that it can be used within init for logic in defining the
widget's contents?
Root Cause
The load_kv() function is called from run(), therefore,
any widget whose styling is defined in this kv file and is created
before run() is called (e.g. in __init__), won’t have its styling
applied. Note that build() is called after load_kv has been
called.
Solution
Use Kivy Clock schedule_once() method to invoke the print() function after Kivy completed styling.
Snippets
def __init__(self, **kwargs):
super(Customwidget, self).__init__(**kwargs)
Clock.schedule_once(lambda dt: print(f"mystring={self.mystring}"), 0.02)
Example
main.py
from kivy.base import runTouchApp
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import StringProperty
from kivy.clock import Clock
class Customwidget(BoxLayout):
mystring = StringProperty()
def __init__(self, **kwargs):
super(Customwidget, self).__init__(**kwargs)
Clock.schedule_once(lambda dt: print(f"mystring={self.mystring}"), 0.02)
runTouchApp(Builder.load_string("""
Screen:
BoxLayout:
orientation: 'vertical'
Customwidget:
mystring: 'stringcontents'
"""))