User-friendly exception if root flow does not override the run() method

src/lightning_app/CHANGELOG.md

@@ -22,6 +22,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Improve Lightning App connect logic by disconnecting automatically ([#14532](https://github.com/Lightning-AI/lightning/pull/14532))
 
 
+- Improved the error messsage when the root `LightningFlow` passed to `LightningApp` is missing the `run` method ([#14760](https://github.com/Lightning-AI/lightning/pull/14760))
+
+
+
 ### Fixed
 
 - Making threadpool non default from LightningCloud client  ([#14757](https://github.com/Lightning-AI/lightning/pull/14757))

src/lightning_app/core/app.py

@@ -26,7 +26,7 @@
 from lightning_app.storage.path import storage_root_dir
 from lightning_app.utilities.app_helpers import _delta_to_app_state_delta, _LightningAppRef, Logger
 from lightning_app.utilities.commands.base import _process_requests
-from lightning_app.utilities.component import _convert_paths_after_init
+from lightning_app.utilities.component import _convert_paths_after_init, _validate_root_flow
 from lightning_app.utilities.enum import AppStage, CacheCallsKeys
 from lightning_app.utilities.exceptions import CacheMissException, ExitAppException
 from lightning_app.utilities.layout import _collect_layout
@@ -58,8 +58,8 @@ def __init__(
         the :class:`~lightning.app.core.flow.LightningFlow` provided.
 
         Arguments:
-            root: The root LightningFlow component, that defined all
-                the app's nested components, running infinitely.
+            root: The root LightningFlow component, that defines all the app's nested components, running infinitely.
+                It must define a `run()` method that the app can call.
             debug: Whether to activate the Lightning Logger debug mode.
                 This can be helpful when reporting bugs on Lightning repo.
 
@@ -77,6 +77,7 @@ def __init__(
             Hello World!
         """
 
+        _validate_root_flow(root)
         self._root = root
 
         # queues definition.

src/lightning_app/utilities/component.py

@@ -5,6 +5,7 @@
 from deepdiff.helper import NotPresent
 from lightning_utilities.core.apply_func import apply_to_collection
 
+from lightning_app.utilities.app_helpers import is_overridden
 from lightning_app.utilities.enum import ComponentContext
 from lightning_app.utilities.tree import breadth_first
 
@@ -118,3 +119,13 @@ def _context(ctx: str) -> Generator[None, None, None]:
     _set_context(ctx)
     yield
     _set_context(prev)
+
+
+def _validate_root_flow(flow: "LightningFlow") -> None:
+    from lightning_app.core.flow import LightningFlow
+
+    if not is_overridden("run", instance=flow, parent=LightningFlow):
+        raise TypeError(
+            "The root flow passed to `LightningApp` does not override the `run()` method. This is required. Please"
+            f" implement `run()` in your `{flow.__class__.__name__}` class."
+        )

tests/tests_app/core/test_lightning_app.py

@@ -1,6 +1,7 @@
 import logging
 import os
 import pickle
+from re import escape
 from time import sleep
 from unittest import mock
 from unittest.mock import ANY
@@ -32,6 +33,29 @@
 logger = logging.getLogger()
 
 
+def test_lightning_app_requires_root_run_method():
+    """Test that a useful exception is raised if the root flow does not override the run method."""
+
+    with pytest.raises(
+        TypeError, match=escape("The root flow passed to `LightningApp` does not override the `run()` method")
+    ):
+        LightningApp(LightningFlow())
+
+    class FlowWithoutRun(LightningFlow):
+        pass
+
+    with pytest.raises(
+        TypeError, match=escape("The root flow passed to `LightningApp` does not override the `run()` method")
+    ):
+        LightningApp(FlowWithoutRun())
+
+    class FlowWithRun(LightningFlow):
+        def run(self):
+            pass
+
+    LightningApp(FlowWithRun())  # no error
+
+
 class B1(LightningFlow):
     def __init__(self):
         super().__init__()