kivy multiscreen can not find the id

Question

all, i can not pass the goi into the function submit_goi in class VQCIA in my kivy code. i'd appreciate if some one can fix the code. print self.goi.text() sentence reports a problem, and it says AttributeError: 'NoneType' object has no attribute 'text'. that is really weird.

# ---------- vqcia.py  ----------
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ObjectProperty 
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen

Builder.load_string("""
<HelloVQCIA>:
    BoxLayout:
        Button:
            text: "Welcome to VQCIA"
            on_press:
                # You can define the duration of the change
                # and the direction of the slide
                root.manager.transition.direction = 'left'
                root.manager.transition.duration = 1
                root.manager.current = 'screen_two'

<VQCIA>:
    BoxLayout:
        orientation: "vertical"
        goi: g_o_i
        padding: 10
        spacing: 10
        size: 400, 200
        pos: 200, 200
        size_hint:None,None

        BoxLayout:
            Label:
                text: "Enter gene of interest with TAIR ID:"
                font_size: '25sp'

        BoxLayout: 
            TextInput:
                id: g_o_i
                hint_text: 'AT3G20770'
                multiline: False
                font_size: '25sp'

        BoxLayout:
            Button:
                text: "Submit"
                size_hint_x: 15
                on_press: 
                    root.submit_goi()
""")

class HelloVQCIA(Screen):
    pass

class VQCIA(Screen):

    # Connects the value in the TextInput widget to these
    # fields
    goi = ObjectProperty()

    def submit_goi(self):
        print self.goi.text

# The ScreenManager controls moving between screens
screen_manager = ScreenManager()

# Add the screens to the manager and then supply a name
# that is used to switch screens
screen_manager.add_widget(HelloVQCIA(name="screen_one"))
screen_manager.add_widget(VQCIA(name="screen_two"))

class VQCIAApp(App):
    def build(self):
        return screen_manager


dbApp = VQCIAApp()
dbApp.run()

I want to make a multiscreen app and the first page is a welcome page. and the second page is the main page that user can submit a form. thank you.

Answer 1

In kv file, move goi: g_o_i to after class rule, <VQCIA>:

<VQCIA>:
    goi: g_o_i
    BoxLayout:
        orientation: "vertical"