Prefer sending data from the view over retrieving it from the controller.

main.py

@@ -13,6 +13,9 @@ class Classification(Enum):
 
 
 class TriangleApp(App):
+    a = StringProperty('')
+    b = StringProperty('')
+    c = StringProperty('')
     classification = StringProperty()
 
     @staticmethod
@@ -36,15 +39,23 @@ class TriangleApp(App):
         return Classification.SCALENE
 
     def update(self):
-        ids = self.root.ids
         try:
-            self.classification = TriangleApp.classify(int(ids.a.text), int(ids.b.text), int(ids.c.text)).value
+            self.classification = TriangleApp.classify(int(self.a), int(self.b), int(self.c)).value
         except ValueError:
             self.classification = f'{Classification.INVALID.value} (non-integer sides)'
 
     def on_start(self):
         self.update()
 
+    def on_a(self, _, __):
+        self.update()
+
+    def on_b(self, _, __):
+        self.update()
+
+    def on_c(self, _, __):
+        self.update()
+
 
 if __name__ == '__main__':
     app = TriangleApp()

triangle.kv

@@ -13,12 +13,11 @@ BoxLayout:
         Widget:
             size_hint: (0.25, 1.0)
         TextInput:
-            id: a
             multiline: False
             write_tab: False
-            text: ''
+            text: app.a
             font_size: sp(24)
-            on_text: app.update()
+            on_text: app.a = self.text
         Widget:
     BoxLayout:
         orientation: 'horizontal'
@@ -31,12 +30,11 @@ BoxLayout:
         Widget:
             size_hint: (0.25, 1.0)
         TextInput:
-            id: b
             multiline: False
             write_tab: False
-            text: ''
+            text: app.b
             font_size: sp(24)
-            on_text: app.update()
+            on_text: app.b = self.text
         Widget:
     BoxLayout:
         orientation: 'horizontal'
@@ -49,12 +47,11 @@ BoxLayout:
         Widget:
             size_hint: (0.25, 1.0)
         TextInput:
-            id: c
             multiline: False
             write_tab: False
-            text: ''
+            text: app.c
             font_size: sp(24)
-            on_text: app.update()
+            on_text: app.c = self.text
         Widget:
     Widget:
     BoxLayout: