Adding numerical values among widgets on KivyMD

Question

I am building an app which serves as a Travel expense manager. I am currently trying to build a system in which you insert the requested amount on an MDTextField and press a button to confirm request. At the same time, on other widget I will like to be adding up the total amount requested.

i.e. If I write 1000 on the TextField and click on the button 'Ingresar Gasto' the value below the Label 'MONTO TOTAL SOLICITADO' should update and both the target MDTextField and the MDTextField with the origin number should have a correctly formated $ 1,000.00.

The same needs to hold true for all the expansion panels in which I have the same widgets.

I have tried the following:

Kivy code of the expansion panel:

<MyContentAliment>:
    adaptive_height: True
    MDBoxLayout:
        orientation:'horizontal'
        adaptive_height:True
        size_hint_x:self.width
        pos_hint: {"center_x":0.5, "center_y":0.5}
        spacing: dp(10)
        padding_horizontal: dp(10)
        MDLabel:
            text: 'Monto:'
            multiline: 'True'
            halign: 'center'
            pos_hint: {"x":0, "top":0.5}
            size_hint_x: 0.15
            font_style: 'Button'
            font_size: 19

        MDTextField:
            id: monto_aliment_viaje
            hint_text: 'Monto a solicitar'
            pos_hint: {"x":0, "top":0.5}
            halign: 'left'
            size_hint_x: 0.3
            helper_text: 'Ingresar el monto a solicitar'
            helper_text_mode: 'on_focus'
            write_tab: False
            required: True
            on_text:
                root.limit_currency()

        MDRaisedButton:
            id: boton_aliment_viaje
            pos_hint: {"x":0, "top":0.5}
            text:'Ingresar Gasto'
            on_release:
                root.sumar_gasto()

Kivy code of the MDCard (containing the target widget):

<TravelManagerWindow>:
    BoxLayout:
        size_hint:1,0.85
        pos_hint: {"center_x": 0.5, "center_y":0.37}
        adaptive_height:True
        height: self.minimum_height

        ScrollView:
            adaptive_height:True

            GridLayout:
                id: container
                size_hint_y: None
                cols: 1
                row_default_height: root.height*0.10
                height: self.minimum_height 

                MDBoxLayout:
                    adaptive_height: True
                    orientation: 'horizontal'
                    GridLayout:
                        id: panel_container
                        size_hint_x: 0.6
                        cols: 1
                        adaptive_height: True
            
                    MDBoxLayout:
                        size_hint_x: 0.05       
                    MDCard:
                        id: resumen_solicitud
                        size_hint: None, None
                        size: "250dp", "300dp"
                        pos_hint: {"top": 0.9, "center_x": .5}
                        elevation: 0.1

                        MDBoxLayout:
                            orientation: 'vertical'
                            canvas.before:
                                Color:
                                    rgba: 0.8, 0.8, 0.8, 1
                                Rectangle:
                                    pos: self.pos
                                    size: self.size
                            MDLabel:
                                text: 'Monto Total Solicitado'
                                font_style: 'Button'
                                halign: 'center'
                                font_size: (root.width**2 + root.height**2) / 15.5**4
                                size_hint_y: 0.2
                            MDSeparator:
                                height: "1dp"
                            MDTextField:
                                id: suma_solic_viaje
                                text: "$ 0.00"
                                bold: True
                                line_color_normal: app.theme_cls.primary_color
                                halign: "center"
                                size_hint_x: 0.8

Last but not least, the Python code:

class TravelManagerWindow(Screen):
    viajeInicio = ObjectProperty(None)
    panel_container = ObjectProperty(None)
    travel_list = ObjectProperty(None)
    DateMDTextField = ObjectProperty(None)
    menu = ObjectProperty()

    # EXPANSION PANEL PARA SOLICITAR GV
    def set_expansion_panel(self):
        #FOOD PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(icon="food.png", 
                                     content=MyContentAliment(),
                                     panel_cls=MDExpansionPanelOneLine(
                                                 text="Alimentacion")))
        # CASETAS PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(
                               icon="casetas.png", 
                                     content=MyContentCasetas(),
                                           panel_cls=MDExpansionPanelOneLine(
                                                         text="Casetas")))
        # GAS PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(icon="gas.png", 
                          content=MyContentGasolina(),
                                panel_cls=MDExpansionPanelOneLine(
                                                         text="Gasolina")))
        # HOSPEDAJE PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(
                 icon="hospedaje.png", content=MyContentHosped(),                                                         
                                 panel_cls=MDExpansionPanelOneLine(
                                                         text="Hospedaje")))
        # VARIOS PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(
                icon="varios.png", content=MyContentVarios(),                                                  
                        panel_cls=MDExpansionPanelOneLine(
                                                         text="Varios")))

ALL OF THE PANEL CONTAINERS HAVE THE SAME CODE, ONLY REFERENCING TO DIFFERENT WIDGETS (WERE NOT ADDED FOR SIMPLICITY)

class MyContentAliment(BoxLayout):
    def apply_currency_format(self):
        # if len <= 3
        if len(self.ids.monto_aliment_viaje.text) <= 3 and 
                (self.ids.monto_aliment_viaje.text).isnumeric():
            self.ids.monto_aliment_viaje.text = "$" + self.ids.monto_aliment_viaje.text + '.00'
        # n,nnn
        elif len(self.ids.monto_aliment_viaje.text) == 4 and 
                (self.ids.monto_aliment_viaje.text).isnumeric():
            self.ids.monto_aliment_viaje.text = "$" + self.ids.monto_aliment_viaje.text[0] + "," + \
                                            self.ids.monto_aliment_viaje.text[1:] + '.00'
        # nn,nnn
        elif len(self.ids.monto_aliment_viaje.text) == 5 and 
                (self.ids.monto_aliment_viaje.text).isnumeric():
            self.ids.monto_aliment_viaje.text = "$" + self.ids.monto_aliment_viaje.text[:2] + "," + \
                                            self.ids.monto_aliment_viaje.text[2:] + '.00'

    def limit_currency(self):
        if len(self.ids.monto_aliment_viaje.text) > 5 and 
                self.ids.monto_aliment_viaje.text.startswith('$') == False:
            self.ids.monto_aliment_viaje.text = self.ids.monto_aliment_viaje.text[:-1]

    def sumar_gasto(self):
        if self.ids.monto_aliment_viaje.text == "":
            pass
        else:
            travel_manager = TravelManagerWindow()
            monto_total = float(travel_manager.ids.suma_solic_viaje.text[2:])
            monto_total += float(self.ids.monto_aliment_viaje.text)
            travel_manager.ids.suma_solic_viaje.text= "$ " + str(monto_total)
            self.apply_currency_format()

### WINDOW MANAGER ################################

class WindowManager(ScreenManager):
    pass

ScreenManager().add_widget(LoginWindow(name='login'))
ScreenManager().add_widget(CreateAccountWindow(name='create'))
ScreenManager().add_widget(MainWindow(name='main'))
ScreenManager().add_widget(IngActivWindow(name='ingActiv'))
ScreenManager().add_widget(CronogramaWindow(name='cronograma'))
ScreenManager().add_widget(TravelManagerWindow(name='travelManager'))
ScreenManager().add_widget(SoporteTecnicoWindow(name='soporteTecnico'))

class powerApp2(MDApp):
    pass

if __name__ == "__main__":
    powerApp2().run()

Currently I get the correct formats due to the apply_currency_format function. Nevertheless, when I press the button, the value of the target text field remains unchanged.

Thanks a lot in advance.

CODE FOR MINIMAL REPRODUCIBLE EXAMPLE:

Python Code:

from kivy.properties import ObjectProperty
from kivy.uix.screenmanager import ScreenManager, Screen
from kivymd.app import MDApp
from kivymd.uix.expansionpanel import MDExpansionPanel, MDExpansionPanelOneLine
from kivy.uix.boxlayout import BoxLayout


class MyContentAliment(BoxLayout):
    def apply_currency_format(self):
        # if len <= 3
        if len(self.ids.monto_aliment_viaje.text) <= 3 and self.ids.monto_aliment_viaje.text.isnumeric():
            self.ids.monto_aliment_viaje.text = "$" + self.ids.monto_aliment_viaje.text + '.00'
        # n,nnn
        elif len(self.ids.monto_aliment_viaje.text) == 4 and self.ids.monto_aliment_viaje.text.isnumeric():
            self.ids.monto_aliment_viaje.text = "$" + self.ids.monto_aliment_viaje.text[0] + "," + \
                                        self.ids.monto_aliment_viaje.text[1:] + '.00'
        # nn,nnn
        elif len(self.ids.monto_aliment_viaje.text) == 5 and self.ids.monto_aliment_viaje.text.isnumeric():
            self.ids.monto_aliment_viaje.text = "$" + self.ids.monto_aliment_viaje.text[:2] + "," + \
                                        self.ids.monto_aliment_viaje.text[2:] + '.00'

    def limit_currency(self):
        if len(self.ids.monto_aliment_viaje.text) > 5 and self.ids.monto_aliment_viaje.text.startswith('$') == False:
            self.ids.monto_aliment_viaje.text = self.ids.monto_aliment_viaje.text[:-1]

    def sumar_gasto(self):
        if self.ids.monto_aliment_viaje.text == "":
            pass
        else:
            travel_manager = TravelManagerWindow()
            monto_total = float(travel_manager.ids.suma_solic_viaje.text[2:])
            monto_total += float(self.ids.monto_aliment_viaje.text)
            travel_manager.ids.suma_solic_viaje.text = "$ " + str(monto_total)
            self.apply_currency_format()


class MyContentCasetas(BoxLayout):
    def apply_currency_format(self):
        # if len <= 3
        if len(self.ids.monto_casetas_viaje.text) <= 3 and self.ids.monto_casetas_viaje.text.isnumeric():
            self.ids.monto_casetas_viaje.text = "$" + self.ids.monto_casetas_viaje.text + '.00'
        # n,nnn
        elif len(self.ids.monto_casetas_viaje.text) == 4 and self.ids.monto_casetas_viaje.text.isnumeric():
            self.ids.monto_casetas_viaje.text = "$" + self.ids.monto_casetas_viaje.text[0] + "," + \
                                            self.ids.monto_casetas_viaje.text[1:] + '.00'
        # nn,nnn
        elif len(self.ids.monto_casetas_viaje.text) == 5 and self.ids.monto_casetas_viaje.text.isnumeric():
            self.ids.monto_casetas_viaje.text = "$" + self.ids.monto_casetas_viaje.text[:2] + "," + \
                                            self.ids.monto_casetas_viaje.text[2:] + '.00'

    def limit_currency(self):
        if len(self.ids.monto_casetas_viaje.text) > 5 and self.ids.monto_casetas_viaje.text.startswith('$') == False:
            self.ids.monto_casetas_viaje.text = self.ids.monto_casetas_viaje.text[:-1]


class MyContentGasolina(BoxLayout):
    def apply_currency_format(self):
        # if len <= 3
        if len(self.ids.monto_gas_viaje.text) <= 3 and self.ids.monto_gas_viaje.text.isnumeric():
            self.ids.monto_gas_viaje.text = "$" + self.ids.monto_gas_viaje.text + '.00'
        # n,nnn
        elif len(self.ids.monto_gas_viaje.text) == 4 and self.ids.monto_gas_viaje.text.isnumeric():
            self.ids.monto_gas_viaje.text = "$" + self.ids.monto_gas_viaje.text[0] + "," + \
                                        self.ids.monto_gas_viaje.text[1:] + '.00'
        # nn,nnn
        elif len(self.ids.monto_gas_viaje.text) == 5 and self.ids.monto_gas_viaje.text.isnumeric():
            self.ids.monto_gas_viaje.text = "$" + self.ids.monto_gas_viaje.text[:2] + "," + \
                                        self.ids.monto_gas_viaje.text[2:] + '.00'

    def limit_currency(self):
        if len(self.ids.monto_gas_viaje.text) > 5 and self.ids.monto_gas_viaje.text.startswith('$') == False:
            self.ids.monto_gas_viaje.text = self.ids.monto_gas_viaje.text[:-1]


class MyContentHosped(BoxLayout):
    def apply_currency_format(self):
        # if len <= 3
        if len(self.ids.monto_hosped_viaje.text) <= 3 and self.ids.monto_hosped_viaje.text.isnumeric():
            self.ids.monto_hosped_viaje.text = "$" + self.ids.monto_hosped_viaje.text + '.00'
        # n,nnn
        elif len(self.ids.monto_hosped_viaje.text) == 4 and self.ids.monto_hosped_viaje.text.isnumeric():
            self.ids.monto_hosped_viaje.text = "$" + self.ids.monto_hosped_viaje.text[0] + "," + \
                                           self.ids.monto_hosped_viaje.text[1:] + '.00'
        # nn,nnn
        elif len(self.ids.monto_hosped_viaje.text) == 5 and self.ids.monto_hosped_viaje.text.isnumeric():
            self.ids.monto_hosped_viaje.text = "$" + self.ids.monto_hosped_viaje.text[:2] + "," + \
                                           self.ids.monto_hosped_viaje.text[2:] + '.00'

    def limit_currency(self):
        if len(self.ids.monto_hosped_viaje.text) > 5 and self.ids.monto_hosped_viaje.text.startswith('$') == False:
            self.ids.monto_hosped_viaje.text = self.ids.monto_hosped_viaje.text[:-1]


class MyContentVarios(BoxLayout):
    def apply_currency_format(self):
        # if len <= 3
        if len(self.ids.monto_varios_viaje.text) <= 3 and self.ids.monto_varios_viaje.text.isnumeric():
            self.ids.monto_varios_viaje.text = "$" + self.ids.monto_varios_viaje.text + '.00'
        # n,nnn
        elif len(self.ids.monto_varios_viaje.text) == 4 and self.ids.monto_varios_viaje.text.isnumeric():
            self.ids.monto_varios_viaje.text = "$" + self.ids.monto_varios_viaje.text[0] + "," + \
                                           self.ids.monto_varios_viaje.text[1:] + '.00'
        # nn,nnn
        elif len(self.ids.monto_varios_viaje.text) == 5 and self.ids.monto_varios_viaje.text.isnumeric():
            self.ids.monto_varios_viaje.text = "$" + self.ids.monto_varios_viaje.text[:2] + "," + \
                                           self.ids.monto_varios_viaje.text[2:] + '.00'

    def limit_currency(self):
        if len(self.ids.monto_varios_viaje.text) > 5 and self.ids.monto_varios_viaje.text.startswith('$') == False:
            self.ids.monto_varios_viaje.text = self.ids.monto_varios_viaje.text[:-1]


class LoginWindow(Screen):
    pass

class TravelManagerWindow(Screen):
    panel_container = ObjectProperty(None)

    # EXPANSION PANEL PARA SOLICITAR GV
    def set_expansion_panel(self):
        # FOOD PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(icon="food", content=MyContentAliment(),
                                                         panel_cls=MDExpansionPanelOneLine(text="Alimentacion")))
        # CASETAS PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(icon="food", content=MyContentCasetas(),
                                                         panel_cls=MDExpansionPanelOneLine(text="Casetas")))
        # GAS PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(icon="food", content=MyContentGasolina(),
                                                         panel_cls=MDExpansionPanelOneLine(text="Gasolina")))
        # HOSPEDAJE PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(icon="food", content=MyContentHosped(),
                                                         panel_cls=MDExpansionPanelOneLine(text="Hospedaje")))
        # VARIOS PANEL
        self.ids.panel_container.add_widget(MDExpansionPanel(icon="food", content=MyContentVarios(),
                                                         panel_cls=MDExpansionPanelOneLine(text="Varios")))

### WINDOW MANAGER ################################


class WindowManager(ScreenManager):
    pass

ScreenManager().add_widget(LoginWindow(name='login'))
ScreenManager().add_widget(TravelManagerWindow(name='travelManager'))


class reprodExample(MDApp):
    def build(self):
        self.theme_cls.primary_palette = "Teal"
        return WindowManager()


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

Code for KV File:

<WindowManager>:
    LoginWindow:
    TravelManagerWindow:

<LoginWindow>:
    MDRaisedButton:
        text: 'Enter'
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        size_hint: None, None
        on_release:
            root.manager.transition.direction = 'up'
            root.manager.current = 'travelManager'

<TravelManagerWindow>:
    name:'travelManager'
    on_pre_enter: root.set_expansion_panel()

    BoxLayout:
        orientation: 'vertical'
        size_hint:1,0.85
        pos_hint: {"center_x": 0.5, "center_y":0.37}
        adaptive_height:True
        height: self.minimum_height

        ScrollView:
            adaptive_height:True

            GridLayout:
                size_hint_y: None
                cols: 1
                row_default_height: root.height*0.10
                height: self.minimum_height

                BoxLayout:
                    adaptive_height: True
                    orientation: 'horizontal'

                    GridLayout:
                        id: panel_container
                        size_hint_x: 0.6
                        cols: 1
                        adaptive_height: True

                    BoxLayout:
                        size_hint_x: 0.05
                    MDCard:
                        id: resumen_solicitud
                        size_hint: None, None
                        size: "250dp", "300dp"
                        pos_hint: {"top": 0.9, "center_x": .5}
                        elevation: 0.1

                        MDBoxLayout:
                            orientation: 'vertical'
                            canvas.before:
                                Color:
                                    rgba: 0.8, 0.8, 0.8, 1
                                Rectangle:
                                    pos: self.pos
                                    size: self.size
                            MDLabel:
                                text: 'Monto Total Solicitado'
                                font_style: 'Button'
                                halign: 'center'
                                font_size: (root.width**2 + root.height**2) / 15.5**4
                                size_hint_y: 0.2
                            MDSeparator:
                                height: "1dp"
                            MDTextField:
                                id: suma_solic_viaje
                                text: "$ 0.00"
                                bold: True
                                line_color_normal: app.theme_cls.primary_color
                                halign: "center"
                                size_hint_x: 0.8
                                pos_hint: {'center_x': 0.5, 'center_y': 0.5}
                            MDSeparator:
                                height: "1dp"
                            BoxLayout:
                                id: expense_graph
                                halign: 'center'

<MyContentAliment>:
    adaptive_height: True
    MDBoxLayout:
        orientation:'horizontal'
        adaptive_height:True
        size_hint_x:self.width
        pos_hint: {"center_x":0.5, "center_y":0.5}
        spacing: dp(10)
        padding_horizontal: dp(10)
        MDLabel:
            text: 'Monto:'
            multiline: 'True'
            halign: 'center'
            pos_hint: {"x":0, "top":0.5}
            size_hint_x: 0.15
            font_style: 'Button'
            font_size: 19

        MDTextField:
            id: monto_aliment_viaje
            hint_text: 'Monto a solicitar'
            pos_hint: {"x":0, "top":0.5}
            halign: 'left'
            size_hint_x: 0.3
            helper_text: 'Ingresar el monto a solicitar'
            helper_text_mode: 'on_focus'
            write_tab: False
            required: True
            on_text:
                root.limit_currency()

        MDRaisedButton:
            id: boton_aliment_viaje
            pos_hint: {"x":0, "top":0.5}
            text:'Ingresar Gasto'
            on_release:
                root.sumar_gasto()

### CASETAS
<MyContentCasetas>:
    adaptive_height: True
    MDBoxLayout:
        orientation:'horizontal'
        adaptive_height:True
        size_hint_x:self.width
        pos_hint: {"center_x":0.5, "center_y":0.5}
        spacing: dp(10)
        padding_horizontal: dp(10)
        MDLabel:
            text: 'Monto:'
            multiline: 'True'
            halign: 'center'
            pos_hint: {"x":0, "top":0.5}
            size_hint_x: 0.15
            font_style: 'Button'
            font_size: 19

        MDTextField:
            id: monto_casetas_viaje
            hint_text: 'Monto a solicitar'
            pos_hint: {"x":0, "top":0.5}
            halign: 'left'
            size_hint_x: 0.3
            helper_text: 'Ingresar el monto a solicitar'
            helper_text_mode: 'on_focus'
            write_tab: False
            #input_filter: 'float'
            required: True
            on_text:
                root.limit_currency()

        MDRaisedButton:
            id: boton_casetas_viaje
            pos_hint: {"x":0, "top":0.5}
            text:'Ingresar Gasto'
            on_release:
                root.apply_currency_format()

        BoxLayout:
            size_hint_x: 0.05

### GASOLINA
<MyContentGasolina>:
    adaptive_height: True
    MDBoxLayout:
        orientation:'horizontal'
        adaptive_height:True
        size_hint_x:self.width
        pos_hint: {"center_x":0.5, "center_y":0.5}
        spacing: dp(10)
        padding_horizontal: dp(10)
        MDLabel:
            text: 'Monto:'
            multiline: 'True'
            halign: 'center'
            pos_hint: {"x":0, "top":0.5}
            size_hint_x: 0.15
            font_style: 'Button'
            font_size: 19

        MDTextField:
            id: monto_gas_viaje
            hint_text: 'Monto a solicitar'
            pos_hint: {"x":0, "top":0.5}
            halign: 'left'
            size_hint_x: 0.3
            helper_text: 'Ingresar el monto a solicitar'
            helper_text_mode: 'on_focus'
            write_tab: False
            #input_filter: 'float'
            required: True
            on_text:
                root.limit_currency()

        MDRaisedButton:
            id: boton_gas_viaje
            pos_hint: {"x":0, "top":0.5}
            text:'Ingresar Gasto'
            on_release:
                root.apply_currency_format()

        BoxLayout:
            size_hint_x: 0.05

 ### HOSPEDAJE
<MyContentHosped>:
    adaptive_height: True
    MDBoxLayout:
        orientation:'horizontal'
        adaptive_height:True
        size_hint_x:self.width
        pos_hint: {"center_x":0.5, "center_y":0.5}
        spacing: dp(10)
        padding_horizontal: dp(10)
        MDLabel:
            text: 'Monto:'
            multiline: 'True'
            halign: 'center'
            pos_hint: {"x":0, "top":0.5}
            size_hint_x: 0.15
            font_style: 'Button'
            font_size: 19

        MDTextField:
            id: monto_hosped_viaje
            hint_text: 'Monto a solicitar'
            pos_hint: {"x":0, "top":0.5}
            halign: 'left'
            size_hint_x: 0.3
            helper_text: 'Ingresar el monto a solicitar'
            helper_text_mode: 'on_focus'
            write_tab: False
            #input_filter: 'float'
            required: True
            on_text:
                root.limit_currency()

    MDRaisedButton:
        id: boton_hosped_viaje
        pos_hint: {"x":0, "top":0.5}
        text:'Ingresar Gasto'
        on_release:
            root.apply_currency_format()

        BoxLayout:
            size_hint_x: 0.05

### VARIOS
<MyContentVarios>:
    adaptive_height: True
    MDBoxLayout:
        orientation:'horizontal'
        adaptive_height:True
        size_hint_x:self.width
        pos_hint: {"center_x":0.5, "center_y":0.5}
        spacing: dp(10)
        padding_horizontal: dp(10)
        MDLabel:
            text: 'Monto:'
            multiline: 'True'
            halign: 'center'
            pos_hint: {"x":0, "top":0.5}
            size_hint_x: 0.15
            font_style: 'Button'
            font_size: 19

        MDTextField:
            id: monto_varios_viaje
            hint_text: 'Monto a solicitar'
            pos_hint: {"x":0, "top":0.5}
            halign: 'left'
            size_hint_x: 0.3
            helper_text: 'Ingresar el monto a solicitar'
            helper_text_mode: 'on_focus'
            write_tab: False
            #input_filter: 'float'
            required: True
            on_text:
                root.limit_currency()

        MDRaisedButton:
            id: boton_varios_viaje
            pos_hint: {"x":0, "top":0.5}
            text:'Ingresar Gasto'
            on_release:
                root.apply_currency_format()

        BoxLayout:
            size_hint_x: 0.05

Answer 1

In your sumar_gasto() method the line:

travel_manager = TravelManagerWindow()

is creating a new instance of TravelManagerWindow that is unrelated to the instance that is displayed in your GUI. So any changes you make to that instance will have no effect on your GUI.

Without seeing more of your code, I cannot guess how you might access the instance of TravelManagerWindow that is actually in your GUI.

So, either you can figure that out for yourself, or you can post a minimal, complete, verifiable example.

So, with the additional code posted, I think you can replace:

travel_manager = TravelManagerWindow()

with:

travel_manager = MDApp.get_running_app().root.get_screen('travelManager')

This works because the root widget of the MDApp is the WindowManager.

You should also add a name for the LoginWindow in the kv:

<LoginWindow>:
    name: 'login'
    MDRaisedButton:
        text: 'Enter'
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        size_hint: None, None
        on_release:
            root.manager.transition.direction = 'up'
            root.manager.current = 'travelManager'

I also noticed that you have some unnecessary code:

ScreenManager().add_widget(LoginWindow(name='login'))
ScreenManager().add_widget(TravelManagerWindow(name='travelManager'))

The above lines have no effect, for the same reason as did the travel_manager = TravelManagerWindow() code. Calling ScreenManager() creates a new instance of ScreenManager that is not in your GUI, and adding a Widget to that instance will have no effect on your GUI. The rule for WindowManager at the start of the kv will build the two Screens and add then to the WindowManager.