Pass variables from .py to .kv file

Question

I am trying to use variables from python file to .kv file so I searched similar questions and found out the way use Property and coded like this:

# in python file
class Test2App(App):
    abcd = StringProperty('test')
    def build(self):
        return presentation

# in kv file
<MyButton@Button>:
    text: "contents (%s)"%(app.abcd)

    background_color: (255, 255, 255,1)`

and an error comes up.

 AttributeError: 'NoneType' object has no attribute 'bind'
   File "/usr/local/lib/python2.7/dist-packages/kivy/lang/builder.py", line 249, in create_handler
     return eval(value, idmap), bound_list
   File "/root/Desktop/hi/t2.kv", line 61, in <module>
     text: "contents (%s)"%(app.abcd)
   File "/usr/local/lib/python2.7/dist-packages/kivy/lang/parser.py", line 75, in __getattribute__
     object.__getattribute__(self, '_ensure_app')()
   File "/usr/local/lib/python2.7/dist-packages/kivy/lang/parser.py", line 70, in _ensure_app
     app.bind(on_stop=lambda instance:

   File "/usr/local/lib/python2.7/dist-packages/kivy/lang/builder.py", line 615, in _apply_rule
     rctx['ids'])
   File "/usr/local/lib/python2.7/dist-packages/kivy/lang/builder.py", line 254, in create_handler
     cause=tb)

t2.py

#-*- coding: utf-8 -*-
__version__ = "1.0"

import kivy
import os
kivy.require('1.10.0')
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.button import Button
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.gridlayout import GridLayout
from kivy.uix.image import Image
from kivy.uix.label import Label
from kivy.animation import Animation
from kivy.clock import Clock
from kivy.properties import StringProperty
#from kivy.config import Config #windows size fixed
#Config.set('graphics', 'resizable', 0)
from kivy.core.window import Window 
Window.size = (540, 960)
#Window.size = (1080, 1920) 
##########FOR BUS INFORMATION UPDATE#############
from urllib import urlencode, quote_plus
from urllib2 import Request as RQ
from urllib2 import urlopen as UO
import urllib
import xml.etree.ElementTree as etree
import os
import datetime


def oopath(ndid, uor):
    path = os.path.join(ndid + '.txt')
    return path

##############################################################################################################

class StationTest(Screen):

    def __init__(self, **kwargs):
        super(StationTest, self).__init__(**kwargs)

    oo = oopath('TESTTEST', 0) 
    self.rpandgv(oo)

    def rpandgv(self,path): 
    with open(path) as businfo:
        Businfo= [] 
        nolinenum=businfo.readline()
        while nolinenum!='': 
        Businfo.append(nolinenum)
        leftstations = (businfo.readline().rstrip('\n') + ' stations'.rstrip('\n'))
        lefttime = (businfo.readline().rstrip('\n') + ' seconds'.rstrip('\n'))
        nolinenum = businfo.readline().rstrip('\n')
        Businfo.append(leftstations)
        Businfo.append(lefttime)
        self.businfolist = Businfo
        self.lenbil = int(len(Businfo))
        self.numberoflist = int(len(Businfo)/3)




class ScreenManagement(ScreenManager):
    pass

presentation = Builder.load_file("t2.kv")

class Test2App(App):
    abcd = StringProperty('test')
    def build(self):
        return presentation


Test2App().run()

t2.kv

# -*- coding: utf-8 -*-
#:import NoTransition kivy.uix.screenmanager.NoTransition
#:import SlideTransition kivy.uix.screenmanager.SlideTransition
#:import Label kivy.uix.button.Label

ScreenManagement:
    transition: SlideTransition(direction='left')
    StationTest:

<StationTest>: 
    name: 'StationTest'
    canvas:
        Rectangle:
            pos: self.pos
            size: self.size 
            source: 'image/background.png' #backgroundimage
    header: _header
    ScrollView:
        FloatLayout:
            size_hint_y: None
            height: 500
            BoxLayout:
                id: _header
                orientation: 'vertical'
                size_hint: 1, 0.10
                pos_hint: {'top': 1.0}
                anchor: _anchor
                canvas:
                    Color:              
                        rgba: 0.8, 0.6, 0.4, 1.0
                    Rectangle:
                        pos: self.pos
                        size: self.size
                Label:
                    text: "STATION > STATION"
                    font_size: 40
                BoxLayout
                    id: _anchor
                    size_hint_y: 0.3
                    canvas.before:
                        Color:              
                            rgba: 0.3, 0.5, 0.8, 1.0
                        Rectangle:
                            pos: self.pos
                            size: self.size
                    Label:
                        text: "TEST1234"

            BoxLayout:
                orientation: 'vertical'
                size_hint: 1, 0.35
                padding: 0, -200, 0, 0
                MyButton:
                MyButton:
                MyButton:
                MyButton:



<MyButton@Button>:
    text: "contents (%s)"%(app.abcd)

    background_color: (255, 255, 255,1)

Answer 1

There are two solutions to this problem. Please refer to the solutions and example for details.

Solution 1: kv file - Using If statement

The variable, abcd is None when the kv file is parsed. Add if...else... statement to solve the problem.

Snippet - kv file

<MyButton@Button>:
    text: "" if app.abcd is None else "contents (%s)"%(app.abcd)

    background_color: (255, 255, 255,1)    # white background color
    color: 0, 0, 0, 1    # black color text

Solution 2: Python code - Initialize in build() method

Initialize variable, abcd in the build() method.

Snippet - Python code

class TestApp(App):
    abcd = StringProperty('')

    def build(self):
        self.abcd = 'test'
        return ScreenManagement()

Example

main.py

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import StringProperty


class StationTest(Screen):
    pass


class ScreenManagement(ScreenManager):
    pass


class TestApp(App):
    abcd = StringProperty('test')

    def build(self):
        return ScreenManagement()


TestApp().run()

test.kv

#:kivy 1.11.0
#:import SlideTransition kivy.uix.screenmanager.SlideTransition

<ScreenManagement>:
    transition: SlideTransition(direction='left')
    StationTest:

<StationTest>:
    name: 'StationTest'
    BoxLayout:
        orientation: 'vertical'
        size_hint: 1, 0.35
        padding: 0, -200, 0, 0
        MyButton:
        MyButton:
        MyButton:
        MyButton:



<MyButton@Button>:
    # text: "" if app.abcd is None else "contents (%s)"%(app.abcd)
    text: "contents (%s)"%(app.abcd)

    background_color: (1, 1, 1, 1)    # white background
    color: 0, 0, 0, 1    # black color text

Output