Fix failing lightning cli entry point (#18821)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>

src/lightning/__init__.py

@@ -1,5 +1,6 @@
 """Root package info."""
 import logging
+import sys
 
 # explicitly don't set root logger's propagation and leave this to subpackages to manage
 _logger = logging.getLogger(__name__)
@@ -28,3 +29,19 @@ __all__ = [
     "seed_everything",
     "Fabric",
 ]
+
+
+def _cli_entry_point() -> None:
+    from lightning_utilities.core.imports import ModuleAvailableCache, RequirementCache
+
+    if not (
+        ModuleAvailableCache("lightning.app")
+        if RequirementCache("lightning-utilities<0.10.0")
+        else RequirementCache(module="lightning.app")  # type: ignore[call-arg]
+    ):
+        print("The `lightning` command requires additional dependencies: `pip install lightning[app]`")
+        sys.exit(1)
+
+    from lightning.app.cli.lightning_cli import main
+
+    main()

src/lightning/__setup__.py

@@ -115,7 +115,7 @@ def _setup_args() -> Dict[str, Any]:
         "python_requires": ">=3.8",  # todo: take the lowes based on all packages
         "entry_points": {
             "console_scripts": [
-                "lightning = lightning.app.cli.lightning_cli:main",
+                "lightning = lightning:_cli_entry_point",
             ],
         },
         "setup_requires": [],

tests/tests_fabric/test_cli.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 import contextlib
 import os
+import subprocess
 from io import StringIO
 from unittest import mock
 from unittest.mock import Mock
@@ -20,6 +21,7 @@ from unittest.mock import Mock
 import pytest
 import torch.distributed.run
 from lightning.fabric.cli import _get_supported_strategies, _run_model
+from lightning_utilities.core.imports import ModuleAvailableCache
 
 from tests_fabric.helpers.runif import RunIf
 
@@ -172,3 +174,15 @@ def test_cli_torchrun_num_processes_launched(_, devices, expected, monkeypatch,
             fake_script,
         ]
     )
+
+
+@pytest.mark.skipif("lightning.fabric" == "lightning_fabric", reason="standalone package")
+def test_cli_through_lightning_entry_point():
+    result = subprocess.run("lightning run model --help", capture_output=True, text=True, shell=True)
+    if not ModuleAvailableCache("lightning.app"):
+        message = "The `lightning` command requires additional dependencies"
+        assert message in result.stdout or message in result.stderr
+        assert result.returncode != 0
+    else:
+        message = "Usage: lightning run model [OPTIONS] SCRIPT [SCRIPT_ARGS]"
+        assert message in result.stdout or message in result.stderr