Revised code to avoid student confusion from last semester.

demo.kv

@@ -17,10 +17,10 @@ BoxLayout:
             on_press: app.print_total()
 
 <Field>:
-    BoxLayout:
-        orientation: 'horizontal'
-        Label:
-            text: root.label_text
-        TextInput:
-            id: input
-            multiline: False
+    orientation: 'horizontal'  # Field is a kind of BoxLayout
+    Label:
+        text: root.label_text  # Refers to a property declared in the Field class
+    TextInput:
+        id: input
+        multiline: False
+        write_tab: False

main.py

@@ -4,10 +4,7 @@ from kivy.properties import StringProperty
 
 
 class Field(BoxLayout):
-    label_text = StringProperty('Data: ')
-
-    def __init__(self, **kwargs):
-        super().__init__(**kwargs)
+    label_text = StringProperty()
 
 
 class DemoApp(App):
@@ -26,7 +23,7 @@ class DemoApp(App):
         total = 0
         for field in container.children:
             try:
-                total += int(field.ids.input.text)
+                total += float(field.ids.input.text)  # refer to a subwidget by id
             except ValueError:
                 pass  # ignore non-numeric values
         print('Total of fields\' numeric values: {total}'.format(total=total))