How to use the kivy.properties.StringProperty function in Kivy

halign: 'center'
        Widget:
            size_hint_y: None
            height: 1000
            on_touch_down: print 'touched at', args[-1].pos

:
    title: 'Error' 
    size_hint: 0.75, 0.75
    Label:
        text: root.error_text
'''


class ErrorPopup(Popup):
    error_text = StringProperty('')

def raise_error(error):
    print('ERROR:',  error)
    ErrorPopup(error_text=error).open()

class TestApp(App):
    def build(self):
        root = Builder.load_string(kv)
        Clock.schedule_interval(self.print_something, 2)
        # Clock.schedule_interval(self.test_pyjnius, 5)
        print('testing metrics')
        from kivy.metrics import Metrics
        print('dpi is', Metrics.dpi)
        print('density is', Metrics.density)
        print('fontscale is', Metrics.fontscale)
        return root

def on_touch_down(self, touch):
        """
        To understand the image movement, to left or to right;
        previous position must be kept and keep updating.
        """
        super(MyScatterLayout, self).on_touch_down(touch)
        self.pre_posx = touch.pos[0]
        self.grab_posx = self.pos[0]


class Pomodoro(BoxLayout):
    time_period = NumericProperty()
    rotation = NumericProperty(0)
    sprint_count = NumericProperty(0)

    time_display = StringProperty()
    time_minute = StringProperty()
    time_second = StringProperty()
    state = StringProperty()
    start = StringProperty()
    stop = StringProperty()

    logs = ListProperty([])

    server_url = StringProperty()
    #"http://172.18.140.79:8000/api/v1.0/put/pomodoro"
    server_user = StringProperty("barbaros")

    server_send = BooleanProperty(False)
    count_start = BooleanProperty(False)
    connect_server = BooleanProperty(False)

def __init__(self, **kwargs):
        self.transition = NoTransition()
        super(Main, self).__init__()
        if str(platform) == 'linux':
            app.config.set('accessibility', 'tts', 0)

class MainApp(App):
    pixel = 'assets/img/pixel.png'
    use_kivy_settings = False
    backgrounds = ListProperty()
    current_background = StringProperty()
    current_search = StringProperty()
    selected_id = NumericProperty()
    folder = StringProperty()
    data_folder = StringProperty()
    notifications_queue = []

    def on_selected_id(self, *args):
        """Used to store the currently viewed/chosen aphorism id"""
        id = int(self.selected_id)
        if id > 0:
            actions = app.root.ids.FormList.ids.form_list_actions
            actions.clear_widgets()
            actions.add_widget(FormListActions())

    def __init__(self):
        self.title = 'Pocket Philosopher'
        self.icon = 'assets/img/icon.png'
        self.folder = os.path.dirname(os.path.abspath(__file__))
        # make sure android uses the /sdcard folder instead of data/
        if platform == 'android':

import global_variables

class FrontPage(Screen, MakesmithInitFuncs):
    textconsole    = ObjectProperty(None)
    connectmenu    = ObjectProperty(None) #make ConnectMenu object accessible at this scope
    gcodecanvas    = ObjectProperty(None) 
    screenControls = ObjectProperty(None) 
    
    connectionStatus = StringProperty("Not Connected")
    
    xReadoutPos = StringProperty("0 mm")
    yReadoutPos = StringProperty("0 mm")
    zReadoutPos = StringProperty("0 mm")
    ReadoutVel = StringProperty(" 0 mm/m")
    gcodeVel = StringProperty(" 0 mm/m")
    percentComplete = StringProperty("0.0%")
    
    numericalPosX  = 0.0
    numericalPosY  = 0.0

    previousPosX = 0.0
    previousPosY = 0.0   

    lastpos=(0,0,0)
    lasttime=0.0
    tick=0
    
    stepsizeval  = 0
    
    consoleText  = StringProperty(" ")
    
    units = StringProperty("MM")

Builder.load_string('''
:
    auto_dismiss: True
    size_hint: None, None
    size: 400, 200
    on_open: root.dismiss_trigger()
    title: root.title
    Label:
        text: root.text
''')


class InformationPopup(Popup):
    title = StringProperty('Information')
    text = StringProperty('')

    def __init__(self, time=1.5, **kwargs):
        super(InformationPopup, self).__init__(**kwargs)
        self.dismiss_trigger = Clock.create_trigger(self.dismiss, time)


Factory.register('InformationPopup', cls=InformationPopup)

text: '1, 1, 1, 1'
            state: 'down'
            on_release: win.shape_color_key = [1, 1, 1, 1]
        ToggleButton:
            group: 'colorkey'
            text: '0, 0, 0, 1'
            on_release: win.shape_color_key = [0, 0, 0, 1]
''')


class Root(BoxLayout):
    pass


class ShapedWindow(App):
    shape_image = StringProperty('', force_dispatch=True)

    def on_shape_image(self, instance, value):
        if 'kivy-icon' in value:
            Window.size = (512, 512)
            Window.shape_image = self.alpha_shape
        else:
            Window.size = (800, 600)
            Window.shape_image = self.default_shape

    def build(self):
        self.default_shape = default_shape
        self.alpha_shape = alpha_shape

        return Root()

# when the label changes size or position
        if self.state == "down":
            self.tab_bar.update_indicator(self.x, self.width)


class MDTabsBase(Widget):
    """
    MDTabsBase allow you to create a tab.
    You must create a new class that inherits from MDTabsBase.
    In this way you have total control over the views of your tabbed panel.
    """

    text = StringProperty()
    """It will be the label text of the tab."""

    icon = StringProperty()
    """It will be the icon of the tab."""

    tab_label = ObjectProperty()
    """It is the label object reference of the tab."""

    def __init__(self, **kwargs):
        self.tab_label = MDTabsLabel(tab=self)
        super().__init__(**kwargs)

    def on_text(self, widget, text):
        # Set the icon
        if text in md_icons:
            self.tab_label.font_name = fonts_path + "materialdesignicons-webfont.ttf"
            self.tab_label.text = md_icons[self.text]
            self.tab_label.font_size = "24sp"
        # Set the label text

class ObjectRenderer(Widget):
    scene = StringProperty('')
    obj_id = StringProperty('')
    obj_translation = ListProperty([0, 0, 0])
    obj_rotation = ListProperty([0, 0, 0])
    obj_scale = NumericProperty(1)
    obj_texture = StringProperty('')
    texture = ObjectProperty(None, allownone=True)
    cam_translation = ListProperty([0, 0, 0])
    cam_rotation = ListProperty([0, 0, 0])
    display_all = BooleanProperty(False)
    light_sources = DictProperty()
    ambiant = NumericProperty(.5)
    diffuse = NumericProperty(.5)
    specular = NumericProperty(.5)
    mode = StringProperty('triangles')

    def __init__(self, **kwargs):
        self.canvas = Canvas()
        with self.canvas:
            self.fbo = Fbo(size=self.size,
                           with_depthbuffer=True,
                           compute_normal_mat=True,
                           clear_color=(0., 0., 0., 0.))

            self.viewport = Rectangle(size=self.size, pos=self.pos)

        self.fbo.shader.source = resource_find(
            join(dirname(__file__), 'simple.glsl'))
        super(ObjectRenderer, self).__init__(**kwargs)

    def on_obj_rotation(self, *args):

def checkCurrent(self, current):
        """Sets the 'current' property to True if the current player is the
           same as the instance reference.
        """
        self.current = self.ref == current

    def on_press(self, *args):
        """Tells the main screen that we want to control the selected
           player.
        """
        self.basescreen.changePlayer(self.ref)


class SqueezePlaylistItem(ButtonBehavior, BoxLayout):
    """Class object for displaying playlist items."""
    artwork = StringProperty("images/10x10_transparent.png")
    artist = StringProperty("Loding playlist")
    trackname = StringProperty("Loding playlist")
    posnum = StringProperty("")
    current = BooleanProperty(False)

    def __init__(self, **kwargs):
        super(SqueezePlaylistItem, self).__init__(**kwargs)
        # Create references to underlying objects
        self.player = kwargs["player"]
        self.np = kwargs["np"]

        # Set the display properties
        try:
            self.artwork = kwargs["track"]["art"]
            self.artist = kwargs["track"]["artist"]
            self.trackname = kwargs["track"]["title"]