[App] Add automatic conversion to structures (#15961)

Author: tchaton

src/lightning_app/CHANGELOG.md

@@ -28,6 +28,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Added a `configure_layout` method to the `LightningWork` which can be used to control how the work is handled in the layout of a parent flow ([#15926](https://github.com/Lightning-AI/lightning/pull/15926))
 
+- Added automatic conversion of list and dict of works and flows to structures ([#15961](https://github.com/Lightning-AI/lightning/pull/15961))
+
 
 ### Changed
 

src/lightning_app/core/flow.py

@@ -142,6 +142,14 @@ def __setattr__(self, name: str, value: Any) -> None:
                 if name in self._works and value != getattr(self, name):
                     raise AttributeError(f"Cannot set attributes as the work can't be changed once defined: {name}")
 
+            if isinstance(value, (list, dict)) and value:
+                _type = (LightningFlow, LightningWork, List, Dict)
+                if isinstance(value, list) and all(isinstance(va, _type) for va in value):
+                    value = List(*value)
+
+                if isinstance(value, dict) and all(isinstance(va, _type) for va in value.values()):
+                    value = Dict(**value)
+
             if isinstance(value, LightningFlow):
                 self._flows.add(name)
                 _set_child_name(self, value, name)
@@ -163,10 +171,10 @@ def __setattr__(self, name: str, value: Any) -> None:
                 value._register_cloud_compute()
 
             elif isinstance(value, (Dict, List)):
-                value._backend = self._backend
                 self._structures.add(name)
                 _set_child_name(self, value, name)
-                if self._backend:
+                if getattr(self, "_backend", None) is not None:
+                    value._backend = self._backend
                     for flow in value.flows:
                         LightningFlow._attach_backend(flow, self._backend)
                     for work in value.works:

tests/tests_app/structures/test_structures.py

@@ -518,3 +518,29 @@ def __init__(self):
     LightningApp(flow)  # wrap in app to init all component names
     assert flow.list_structure[0].name == "root.list_structure.0"
     assert flow.dict_structure["dict_child"].name == "root.dict_structure.dict_child"
+
+
+class FlowWiStructures(LightningFlow):
+    def __init__(self):
+        super().__init__()
+
+        self.ws = [EmptyFlow(), EmptyFlow()]
+
+        self.ws1 = {"a": EmptyFlow(), "b": EmptyFlow()}
+
+        self.ws2 = {
+            "a": EmptyFlow(),
+            "b": EmptyFlow(),
+            "c": List(EmptyFlow(), EmptyFlow()),
+            "d": Dict(**{"a": EmptyFlow()}),
+        }
+
+    def run(self):
+        pass
+
+
+def test_flow_without_structures():
+
+    flow = FlowWiStructures()
+    assert isinstance(flow.ws, List)
+    assert isinstance(flow.ws1, Dict)