kivy,,, assigning global variable

[Sadaka Technology]

class main(..):

    def init.....:
        super.....
        button = Button(on_release=self.on_click)

    def on_click(self,screen,*args)
          self.clear_widgets()
          if screen == '2nd':
              float = 2nd()
          elif screen == '1st':
              float = 1st()
          self.add_widget(float)

class 2nd(...):
    string_text = StringProperty()
    def init.....:
        super.....
        Text = TextInput(text=self.string_text)
         button=Button(on_release=self.on_click)

    def on_click
          self.parent.on_click('1st')

class 1st(...):
     ............


#my only issue is in class 2nd() how can I change the value of the variable string_text, if I am clearing widgets and adding widget again in parent class ( main()), then string_text will again equal to empty ,   I tried to add string_text variable in main() but the same issue I am getting, how can I save the string_text in 2nd() if am removing it and adding it again ?