Support overridden dataclass defaults in control encoding (#5861)

Added logic to retrieve default values from base dataclasses when encoding controls, ensuring overridden defaults in subclasses are correctly emitted. Includes new tests and example usage with custom button controls.

Author: FeodorFitsner

sdk/python/examples/apps/custom_controls/custom_buttons.py

@@ -0,0 +1,45 @@
+from dataclasses import dataclass, field
+from typing import Any
+
+import flet as ft
+
+
+def main(page: ft.Page):
+    @ft.control
+    class MyButton(ft.Button):
+        expand: int = field(default_factory=lambda: 1)
+        style: ft.ButtonStyle = field(
+            default_factory=lambda: ft.ButtonStyle(
+                shape=ft.RoundedRectangleBorder(radius=10)
+            )
+        )
+        bgcolor: ft.Colors = ft.Colors.BLUE_ACCENT
+        icon: Any = ft.Icons.HEADPHONES
+
+    @dataclass
+    class MyButton2(ft.Button):
+        expand: Any = 1
+        bgcolor: ft.Colors = ft.Colors.GREEN_ACCENT
+        style: ft.ButtonStyle = field(
+            default_factory=lambda: ft.ButtonStyle(
+                shape=ft.RoundedRectangleBorder(radius=20)
+            )
+        )
+        icon: ft.IconDataOrControl = ft.Icons.HEADPHONES
+
+    @ft.control
+    class MyButton3(ft.Button):
+        def init(self):
+            self.expand = 1
+            self.bgcolor = ft.Colors.RED_ACCENT
+            self.style = ft.ButtonStyle(shape=ft.RoundedRectangleBorder(radius=30))
+            self.icon = ft.Icons.HEADPHONES
+
+    page.add(
+        ft.Row([MyButton(content="1")]),
+        ft.Row([MyButton2(content="2")]),
+        ft.Row([MyButton3(content="3")]),
+    )
+
+
+ft.run(main)

sdk/python/packages/flet/src/flet/messaging/protocol.py

@@ -8,6 +8,20 @@
 from flet.controls.duration import Duration
 
 
+def _get_root_dataclass_field(cls, field_name):
+    """
+    Returns the field definition from the earliest dataclass in the MRO
+    that declares `field_name`. This lets us recover defaults configured
+    on base controls before subclasses override them.
+    """
+
+    for base in reversed(cls.__mro__):
+        dataclass_fields = getattr(base, "__dataclass_fields__", None)
+        if dataclass_fields and field_name in dataclass_fields:
+            return dataclass_fields[field_name]
+    return None
+
+
 def configure_encode_object_for_msgpack(control_cls):
     def encode_object_for_msgpack(obj):
         if is_dataclass(obj):
@@ -36,10 +50,18 @@ def encode_object_for_msgpack(obj):
                 elif is_dataclass(v):
                     r[field.name] = v
                     prev_classes[field.name] = v
-                elif v is not None and (
-                    v != field.default or not isinstance(obj, control_cls)
-                ):
-                    r[field.name] = v
+                else:
+                    default_value = field.default
+                    if isinstance(obj, control_cls):
+                        root_field = _get_root_dataclass_field(
+                            obj.__class__, field.name
+                        )
+                        if root_field is not None:
+                            default_value = root_field.default
+                    if v is not None and (
+                        v != default_value or not isinstance(obj, control_cls)
+                    ):
+                        r[field.name] = v
 
             if not hasattr(obj, "_frozen"):
                 setattr(obj, "__prev_lists", prev_lists)

sdk/python/packages/flet/tests/test_patch_dataclass.py

@@ -2,7 +2,7 @@
 
 import msgpack
 
-from flet.controls.base_control import BaseControl
+from flet.controls.base_control import BaseControl, control
 from flet.controls.base_page import PageMediaData
 from flet.controls.object_patch import ObjectPatch
 from flet.controls.padding import Padding
@@ -37,6 +37,21 @@ class AppSettings:
     assert settings.config.timeout == 2.5
 
 
+def test_encode_emits_overridden_defaults():
+    @control("BaseTestControl")
+    class BaseTestControl(BaseControl):
+        foo: int = 0
+
+    @control("ChildTestControl")
+    class ChildTestControl(BaseTestControl):
+        foo: int = 5
+
+    encoder = configure_encode_object_for_msgpack(BaseControl)
+    encoded = encoder(ChildTestControl())
+
+    assert encoded["foo"] == 5
+
+
 def test_page_patch_dataclass():
     conn = Connection()
     conn.pubsubhub = PubSubHub()