NameError: name 'fro' not defined

Question

I have the code shown below. In the function populate I have the variables fro, to and transport. I am trying to call these variables into the get function thereafter call them when a Popup is display. These variables will then be displayed in the popup. However, I am having a NameError: name 'fro' is not defined with my code. When i use num = Connected.get() instead of num = g and comment out c =Connected() and g= c.get() i get <bound method MessageBox.get of <connected.MessageBox object at 0x000000000B36F3F0>> displayed in the MessageBox Popup.I don't know what I am doing wrong. Is there a better way to do it?

    class Connected(Screen):

    def populate(self, transText, beginText, toText):
        global fro
        global to
        global transport

        self.rv.data = []
        self.ids.results.text=""
        self.ids.no_entry.text=""

        fro = beginText
        to = toText
        transport = transText        


    def get(self):
        b = fro
        a = StringProperty(b)
        return a

class MessageBox(Popup):
    c = Connected()
    g = c.get()

    route = 'MessageBox'

    num = g
    #num = Connected.get() 

    def popup_dismiss(self):
        self.dismiss()

Kivy.kv

<MessageBox>:
    title: root.route
    size_hint: None, None
    size: 400, 400
    separator_color: 1,0,0,1

    BoxLayout:
        orientation: 'vertical'
        Label:
            id: info
            #text: 'nice'
            text: str(root.num)
        Button:
            size_hint: 1, 0.2
            text: 'OK'
            on_press:
                root.dismiss()

Answer 1

Global variables must be defined after the import afterstatements. the import Since I don't have your complete codes. The example below is just an illustration.

Example

main.py

from kivy.app import App
from kivy.uix.screenmanager import Screen
from kivy.uix.popup import Popup
from kivy.properties import StringProperty


fro = "fro-1-2-3"
to = "to-1-2-3"
transport = "transport-1-2-3"


class Connected(Screen):

    def __init__(self, **kwargs):
        super(Connected, self).__init__(**kwargs)
        self.populate("transText", "beginText", "toText")

    def populate(self, transText, beginText, toText):
        global fro
        global to
        global transport

        # self.rv.data = []
        # self.ids.results.text = ""
        # self.ids.no_entry.text = ""

        fro = beginText
        to = toText
        transport = transText

    def get(self):
        print("Connected.get: from={}".format(fro))
        b = fro
        a = StringProperty(b)
        return a


class MessageBox(Popup):
    c = Connected()
    g = c.get()
    print("MessageBox: g={}".format(g))

    route = 'MessageBox'

    num = g
    #num = Connected.get()

    def popup_dismiss(self):
        self.dismiss()


class TestApp(App):
    def build(self):
        return MessageBox()


if __name__ == "__main__":
    TestApp().run()

test.kv

#:kivy 1.10.0

<Connected>:

<MessageBox>:
    title: root.route
    size_hint: None, None
    size: 400, 400
    separator_color: 1,0,0,1

    BoxLayout:
        orientation: 'vertical'
        Label:
            id: info
            #text: 'nice'
            text: str(root.num)
        Button:
            size_hint: 1, 0.2
            text: 'OK'
            on_press:
                root.dismiss()

Output