[App] Enable properties for the Lightning flow (#15750)

(cherry picked from commit 5cfb176)

src/lightning_app/CHANGELOG.md

@@ -47,6 +47,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed catimage import ([#15712](https://github.com/Lightning-AI/lightning/pull/15712))
 - Parse all lines in app file looking for shebangs to run commands ([#15714](https://github.com/Lightning-AI/lightning/pull/15714))
 
+- Fixed setting property to the LightningFlow ([#15750](https://github.com/Lightning-AI/lightning/pull/15750))
+
+
 
 ## [1.8.1] - 2022-11-10
 

src/lightning_app/core/flow.py

@@ -110,7 +110,11 @@ def name(self):
         """Return the current LightningFlow name."""
         return self._name or "root"
 
-    def __setattr__(self, name, value):
+    def __setattr__(self, name: str, value: Any) -> None:
+        attr = getattr(self.__class__, name, None)
+        if isinstance(attr, property) and attr.fset is not None:
+            return attr.fset(self, value)
+
         from lightning_app.structures import Dict, List
 
         if (

tests/tests_app/core/test_lightning_app.py

@@ -1108,3 +1108,36 @@ def test_cloud_compute_binding():
 
     with pytest.raises(Exception, match="A Cloud Compute can be assigned only to a single Work"):
         FlowCC()
+
+
+class FlowValue(LightningFlow):
+    def __init__(self):
+        super().__init__()
+        self._value = None
+        self._has_found = False
+
+    @property
+    def value(self):
+        return self._value
+
+    @value.setter
+    def value(self, value):
+        self._value = value
+
+    def run(self):
+        if self.value is None:
+            self.value = True
+
+    def __setattr__(self, name, value):
+        if name == "_value" and value is True:
+            self._has_found = True
+        super().__setattr__(name, value)
+
+
+def test_lightning_flow_properties():
+    """Validates setting properties to the LightningFlow properly calls property.fset."""
+
+    flow = FlowValue()
+    assert not flow._has_found
+    flow.run()
+    assert flow._has_found