kivy, Python, when running, nothing on the screen

 from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
import threading
from kivy.core.window import Window

Class Top_Screen (Screen):
    """Top Screen"""

    def press_enter_button(self):
        sm.add_widget(Search_Screen(name='search'))# Add screen just before transition to move initial processing
        sm.remove_widget(self)
        sm.current='search'# transition

class Search_Screen (Screen):
    """search screen"""

    def__init__(self,**kwargs):
        super(Search_Screen,self).__init__(**kwargs)
        self.text='# Get search keywords
        self.btn_mythread=threading.Thread(target=self.search_to_uniprot_thread)

    def press_search_button(self):
        # Start thread when button is pressed

        print ("begin")
        sm.add_widget(Search_Screen(name='wait'))# Add screen just before transition to move initial processing
        sm.current='wait'# transition
        self.btn_mythread.start()#Threadstart


    def search_to_uniprot_thread(self):
            # Connect to Uniprot

            from bioservices import UniProt
            service=UniProt()

            # Replace search value as query

            query=self.ids ["text_box"].text

            # extract and output data
            result=service.search("keyword:"+query)
            print(result)
            print("finish")

            sm.add_widget(Output_Screen(name='output')))# Add screen just before transition to move initial processing
            sm.current='output'

class Wait_Screen (Screen):
    """Wait screen during data extraction"""

    def press_cancel_button(self):
        # Return to Search screen when button is pressed

        sm.remove_widget(self)
        sm.current='search'


classOutput_Screen (Screen):
    """output screen"""

    def press_return_button(self):
        # Return to Search screen when button is pressed

        temp=sm.get_screen('select')
        sm.remove_widget(temp)
        sm.remove_widget(self)
        sm.current='search'

sm=ScreenManager()#ScreenManager


class MobiDBApp (App):
    def build (self):
        sm.add_widget(Top_Screen(name='top')
        sm.current='top'

        return sm

if__name__=='__main__':
    Window.size= (400,220)
    MobiDBApp().run()

<Top_Screen>:
    BoxLayout:
        size —root.size

        Button:
            size:50,50
            size_hint —None, None
            text: 'To Search Screen'
            on_press —Root.press_enter_button()#Top_Screen method is executed

<Search_Screen>:
    BoxLayout:
        orientation:'vertical'
        size —root.size

        Label:
            text_size —self.size
            text: 'Searching in MobiDB'

            align:'left'
            size_hint_x:1
            size_hint_y —0.5
            font_size —24

        Label:
            size_hint_y —0.1
            text:'

        BoxLayout:
            size_hint_x —0.9
            size_hint_y —0.4
            orientation:'horizontal'

            TextInput:
                id:text_box
                font_size —22
                focus —True
                on_text_validate —root.btnClicked_text()

        Label:
            size_hint_y —0.1
            text:'

        FloatLayout:
            Button:
                text: "Search"
                size_hint_x —0.23
                size_hint_y —0.3
                pos_hint: {"x": 0.75, "y":0}
                font_size —20

                on_press:
                    root.manager.transition.direction='RiseInTransition'
                    root.press_search_button()

<Wait_Screen>:
    BoxLayout:
        orientation:'vertical'
        size —root.size

        Label:
            size_hint_y —0.1
            text:'

        Label:
            text_size —self.size
            text: 'Searching Now'

            align:'left'
            size_hint_x:1
            size_hint_y —0.8
            font_size —24

        Label:
            size_hint_y —0.1
            text:'

        Button:
            size:50,50
            size_hint —None, None
            text:"cancel"
            on_press —The method root.press_cancel_button()#Top_Screen is executed.

<Output_Screen>:
        BoxLayout:
        size —root.size

            Button:
                size:50,50
                size_hint —None, None
                text:'finish'
                on_press —The method root.press_return_button()#Top_Screen is executed.