[CLI] Add support for `--key.help=class` (#10767)

CHANGELOG.md

@@ -184,6 +184,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+- Fixed support for `--key.help=class` with the `LightningCLI` ([#10767](https://github.com/PyTorchLightning/pytorch-lightning/pull/10767))
+
+
 - Fixed `_compare_version` for python packages ([#10762](https://github.com/PyTorchLightning/pytorch-lightning/pull/10762))
 
 

pytorch_lightning/utilities/cli.py

@@ -263,9 +263,27 @@ class LightningArgumentParser(ArgumentParser):
             else:
                 clean_argv.append(arg)
             i += 1
+
+        # the user requested a help message
+        help_key = argv_key + ".help"
+        if help_key in passed_args:
+            argv_class = passed_args[help_key]
+            if "." in argv_class:
+                # user passed the class path directly
+                class_path = argv_class
+            else:
+                # convert shorthand format to the classpath
+                for cls in classes:
+                    if cls.__name__ == argv_class:
+                        class_path = _class_path_from_class(cls)
+                        break
+                else:
+                    raise ValueError(f"Could not generate get the class_path for {repr(argv_class)}")
+            return clean_argv + [help_key, class_path]
+
         # generate the associated config file
-        argv_class = passed_args.pop(argv_key, None)
-        if argv_class is None:
+        argv_class = passed_args.pop(argv_key, "")
+        if not argv_class:
             # the user passed a config as a str
             class_path = passed_args[f"{argv_key}.class_path"]
             init_args_key = f"{argv_key}.init_args"
@@ -769,12 +787,16 @@ class LightningCLI:
         return fn_kwargs
 
 
+def _class_path_from_class(class_type: Type) -> str:
+    return class_type.__module__ + "." + class_type.__name__
+
+
 def _global_add_class_path(
     class_type: Type, init_args: Optional[Union[Namespace, Dict[str, Any]]] = None
 ) -> Dict[str, Any]:
     if isinstance(init_args, Namespace):
         init_args = init_args.as_dict()
-    return {"class_path": class_type.__module__ + "." + class_type.__name__, "init_args": init_args or {}}
+    return {"class_path": _class_path_from_class(class_type), "init_args": init_args or {}}
 
 
 def _add_class_path_generator(class_type: Type) -> Callable[[Namespace], Dict[str, Any]]:

requirements/extra.txt

@@ -5,6 +5,6 @@ horovod>=0.21.2  # no need to install with [pytorch] as pytorch is already insta
 torchtext>=0.8.*
 omegaconf>=2.0.5
 hydra-core>=1.0.5
-jsonargparse[signatures]>=4.0.0
+jsonargparse[signatures]>=4.0.4
 gcsfs>=2021.5.0
 rich>=10.2.2

tests/utilities/test_cli.py

@@ -57,7 +57,7 @@ if _TORCHVISION_AVAILABLE:
 
 
 @mock.patch("argparse.ArgumentParser.parse_args")
-def test_default_args(mock_argparse, tmpdir):
+def test_default_args(mock_argparse):
     """Tests default argument parser for Trainer."""
     mock_argparse.return_value = Namespace(**Trainer.default_attributes())
 
@@ -870,7 +870,7 @@ class CustomCallback(Callback):
     pass
 
 
-def test_registries(tmpdir):
+def test_registries():
     assert "SGD" in OPTIMIZER_REGISTRY.names
     assert "RMSprop" in OPTIMIZER_REGISTRY.names
     assert "CustomAdam" in OPTIMIZER_REGISTRY.names
@@ -1360,9 +1360,27 @@ def test_lightning_cli_reinstantiate_trainer():
     assert cli.config_init["trainer"]["max_epochs"] is None
 
 
-def test_cli_configure_optimizers_warning(tmpdir):
+def test_cli_configure_optimizers_warning():
     match = "configure_optimizers` will be overridden by `LightningCLI"
     with mock.patch("sys.argv", ["any.py"]), no_warning_call(UserWarning, match=match):
         LightningCLI(BoringModel, run=False)
     with mock.patch("sys.argv", ["any.py", "--optimizer=Adam"]), pytest.warns(UserWarning, match=match):
         LightningCLI(BoringModel, run=False)
+
+
+def test_cli_help_message():
+    # full class path
+    cli_args = ["any.py", "--optimizer.help=torch.optim.Adam"]
+    classpath_help = StringIO()
+    with mock.patch("sys.argv", cli_args), redirect_stdout(classpath_help), pytest.raises(SystemExit):
+        LightningCLI(BoringModel, run=False)
+
+    cli_args = ["any.py", "--optimizer.help=Adam"]
+    shorthand_help = StringIO()
+    with mock.patch("sys.argv", cli_args), redirect_stdout(shorthand_help), pytest.raises(SystemExit):
+        LightningCLI(BoringModel, run=False)
+
+    # the help messages should match
+    assert shorthand_help.getvalue() == classpath_help.getvalue()
+    # make sure it's not empty
+    assert "Implements Adam" in shorthand_help.getvalue()