How to change a global variable with a Button

Question

I'm working on an program that is suppose to be an assistant for people that go to my local gym. Though I'm having trouble with a global variable difficult to change its string value. I want to be able to press any of the Buttons in main_Screen and have the string of the difficult change. Therefore in the BoulderingDirectory class it can preform the corresponding SQL for my database holding the exercises.

Note: I've cut off a majority of the code because there are multiple sections in the gym and I'm just focusing on this section because it applies exactly the same to other sections.

screen_manager = ScreenManager()

global difficult

difficult = "Beginner"

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

class MyScreenManager(ScreenManager):
   pass

class main_Screen(Screen):
   pass

class urec_facilities(Screen):
   pass

class AdventureDirectory(Screen):
   pass


class BoulderingDirectory(Screen):
    connection = sqlite3.connect("workoutData.db")
    cursor = connection.cursor()

    if (difficult == "Beginner"):
        cursor.execute("SELECT * FROM workoutData WHERE Activity = 
                       'Bouldering' and Difficulty = 'Beginner'")
    elif(difficult == "Intermediate"):
        cursor.execute("SELECT * FROM workoutData WHERE Activity = 
                        'Bouldering' and Difficulty = 'Intermediate'")
    else:
        cursor.execute("SELECT * FROM workoutData WHERE Activity = 
                       'Bouldering' and Difficulty = 'Advanced'")

    rows = StringProperty(str(cursor.fetchall()))


root_widget = Builder.load_string("""


<main_Screen>:
    canvas.before:
        Rectangle:
            pos: self.pos
            size: self.size
            source: "mainScreenImage.png"

Label:
    text: "Select your dificulty"
    font_size: 50
    color: (0.9,0.8,0,1)
    background_color: (0,0,0,0)
    pos_hint: {"x": 0, "y": 0.20}


Button:
    text: 'Beginner'
    font_size: 30
    size_hint: 0.2,0.2
    color: (0.9,0.8,0,1)
    background_color: (0,0,0,0)
    pos_hint: {"x": 0.16, "y": 0.1}
    on_release:
        #set the difficult variable to a string "Beginner"
        app.root.current = 'urecFac'



Button:
    text: 'Intermediate'
    font_size: 30
    size_hint: 0.2,0.2
    color: (0.9,0.8,0,1)
    background_color: (0,0,0,0)
    pos_hint: {"x": 0.41, "y": 0.1}
    on_release:
        #set the difficult variable to a string "Intermediate"
        app.root.current = 'urecFac'


Button:
    text: 'Advanced'
    font_size: 30
    size_hint: 0.2,0.2
    color: (0.9,0.8,0,1)
    background_color: (0,0,0,0)
    pos_hint: {"x": 0.66, "y": 0.1}
    on_release:
        #set the difficult variable to a string "Advanced"
        app.root.current = 'urecFac'

<urec_facilities>:
 BoxLayout:
    orientation: "vertical"
    Button:
        text: "Adventure Center"
        font_size: 50
        color: (0.9,0.8,0,1)
        background_normal: "adventureCenterImage.jpg"
        on_release: app.root.current = "AdventureDirectory"
    Button:
        background_normal: "mainScreenImage.png"
        text: "Back"
        font_size: 50
        color: (0.9,0.8,0,1)
        on_release: app.root.current = "mainScreen"

<AdventureDirectory>:
BoxLayout:
    orientation: "vertical"
    Button:
        text: "Bouldering"
        font_size: 50
        color: (0.9,0.8,0,1)
        on_release:
            app.root.current = "BoulderingDirectory"

    Button:
        text: "Go back"
        font_size: 50
        color: (0.9,0.8,0,1)
        on_release:
            app.root.current = "urecFac"

<BoulderingDirectory>:
BoxLayout:
    orientation: "vertical"
    Label:
        text: root.rows
        text_size: (root.width - 175), None
    Button:
        text: "Go back"
        font_size: 50
        color: (0.9,0.8,0,1)
        background_color: (0,0,0,1)
        on_release:
            app.root.current = "AdventureDirectory"

""")



screen_manager.add_widget(main_Screen(name = "mainScreen"))
screen_manager.add_widget(urec_facilities(name = "urecFac"))
screen_manager.add_widget(AdventureDirectory(name = "AdventureDirectory"))
screen_manager.add_widget(BoulderingDirectory(name = "BoulderingDirectory"))


ScreenManagerApp().run()

Answer 1

It is not necessary or advisable to use global variables. It is not necessary because changing the value of a global variable does not notify the change to the copies and is not recommended because it is difficult to debug (for more information read Why are global variables evil?).

In this case the solution is to use a StringProperty that you can connect to a callback to update the data. On the other hand you must implement the logic of the request within a method so that it can be called several times. I have restructured your project considering the above and the solution is the following:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import Screen
from kivy.properties import StringProperty
import sqlite3

class Main_Screen(Screen):
   difficult = StringProperty()

class Urec_facilities(Screen):
   pass

class AdventureDirectory(Screen):
   pass

class BoulderingDirectory(Screen):
    rows = StringProperty("")

    def load_from_difficult(self, difficult):
        connection = sqlite3.connect("workoutData.db")
        cursor = connection.cursor()
        cursor.execute("SELECT * FROM workoutData WHERE Activity = 'Bouldering' and Difficulty = ?", (difficult,))
        self.rows = str(cursor.fetchall())

root_widget = Builder.load_string("""
ScreenManager:
    Main_Screen:
        name: 'mainScreen'
        on_difficult: bouldering.load_from_difficult(self.difficult)
    Urec_facilities:
        name: 'urecFac'
    AdventureDirectory:
        name: 'AdventureDirectory'
    BoulderingDirectory:
        id: bouldering
        difficult: 'Beginner'
        name: 'BoulderingDirectory'

<Main_Screen>:
    canvas.before:
        Rectangle:
            pos: self.pos
            size: self.size
            source: "mainScreenImage.png"

    Label:
        text: "Select your dificulty"
        font_size: 50
        color: (0.9,0.8,0,1)
        background_color: (0,0,0,0)
        pos_hint: {"x": 0, "y": 0.20}

    Button:
        text: 'Beginner'
        font_size: 30
        size_hint: 0.2,0.2
        color: (0.9,0.8,0,1)
        background_color: (0,0,0,0)
        pos_hint: {"x": 0.16, "y": 0.1}
        on_release:
            root.difficult =  "Beginner"
            app.root.current = 'urecFac'

    Button:
        text: 'Intermediate'
        font_size: 30
        size_hint: 0.2,0.2
        color: (0.9,0.8,0,1)
        background_color: (0,0,0,0)
        pos_hint: {"x": 0.41, "y": 0.1}
        on_release:
            root.difficult = "Intermediate"
            app.root.current = 'urecFac'

    Button:
        text: 'Advanced'
        font_size: 30
        size_hint: 0.2,0.2
        color: (0.9,0.8,0,1)
        background_color: (0,0,0,0)
        pos_hint: {"x": 0.66, "y": 0.1}
        on_release:
            root.difficult = "Advanced"
            app.root.current = 'urecFac'

<Urec_facilities>:
    BoxLayout:
        orientation: "vertical"
        Button:
            text: "Adventure Center"
            font_size: 50
            color: (0.9,0.8,0,1)
            background_normal: "adventureCenterImage.jpg"
            on_release: app.root.current = "AdventureDirectory"
        Button:
            background_normal: "mainScreenImage.png"
            text: "Back"
            font_size: 50
            color: (0.9,0.8,0,1)
            on_release: app.root.current = "mainScreen"

<AdventureDirectory>:
    BoxLayout:
        orientation: "vertical"
        Button:
            text: "Bouldering"
            font_size: 50
            color: (0.9,0.8,0,1)
            on_release:
                app.root.current = "BoulderingDirectory"

        Button:
            text: "Go back"
            font_size: 50
            color: (0.9,0.8,0,1)
            on_release:
                app.root.current = "urecFac"

<BoulderingDirectory>:
    BoxLayout:
        orientation: "vertical"
        Label:
            text: root.rows
            text_size: (root.width - 175), None
        Button:
            text: "Go back"
            font_size: 50
            color: (0.9,0.8,0,1)
            background_color: (0,0,0,1)
            on_release:
                app.root.current = "AdventureDirectory"

""")

class ScreenManagerApp(App):
    def build(self):
        return root_widget

if __name__ == '__main__':
    ScreenManagerApp().run()

Answer 2

Before using a global variable in a class or a function, you have to declare that the variable you are referring to is the global variable or it is created as a local variable for that definition.

So, do

class BoulderingDirectory(Screen):
    global difficult
    ...

Answer 3

The global keyword allows you to create and modify variables at the global scope rather than the local scope. The syntax for it is global variableName. So just add global difficult to any place where you need to change and/or access the global variable.

You can change the beginning of your BoulderingDirectory class to this:

class BoulderingDirectory(Screen):
    global difficult

See here for more info on the global keyword.