Surface Neptune installation problems to the user

src/pytorch_lightning/loggers/neptune.py

@@ -231,11 +231,8 @@ def __init__(
         agg_default_func: Optional[Callable[[Sequence[float]], float]] = None,
         **neptune_run_kwargs: Any,
     ):
-        if neptune is None:
-            raise ModuleNotFoundError(
-                "You want to use the `Neptune` logger which is not installed yet, install it with"
-                " `pip install neptune-client`."
-            )
+        if not _NEPTUNE_AVAILABLE:
+            raise ModuleNotFoundError(str(_NEPTUNE_AVAILABLE))
         # verify if user passed proper init arguments
         self._verify_input_arguments(api_key, project, name, run, neptune_run_kwargs)
         super().__init__(agg_key_funcs=agg_key_funcs, agg_default_func=agg_default_func)

tests/tests_pytorch/loggers/test_all.py

@@ -43,6 +43,7 @@
     mock.patch("pytorch_lightning.loggers.mlflow.mlflow"),
     mock.patch("pytorch_lightning.loggers.mlflow.MlflowClient"),
     mock.patch("pytorch_lightning.loggers.neptune.neptune", new_callable=create_neptune_mock),
+    mock.patch("pytorch_lightning.loggers.neptune._NEPTUNE_AVAILABLE", return_value=True),
     mock.patch("pytorch_lightning.loggers.wandb.wandb"),
     mock.patch("pytorch_lightning.loggers.wandb.Run", new=mock.Mock),
 )
@@ -290,7 +291,9 @@ def test_logger_with_prefix_all(tmpdir, monkeypatch):
         logger.experiment.log_metric.assert_called_once_with(ANY, "tmp-test", 1.0, ANY, 0)
 
     # Neptune
-    with mock.patch("pytorch_lightning.loggers.neptune.neptune"):
+    with mock.patch("pytorch_lightning.loggers.neptune.neptune"), mock.patch(
+        "pytorch_lightning.loggers.neptune._NEPTUNE_AVAILABLE", return_value=True
+    ):
         logger = _instantiate_logger(NeptuneLogger, api_key="test", project="project", save_dir=tmpdir, prefix=prefix)
         assert logger.experiment.__getitem__.call_count == 2
         logger.log_metrics({"test": 1.0}, step=0)

tests/tests_pytorch/loggers/test_neptune.py

@@ -15,6 +15,7 @@
 import pickle
 import unittest
 from collections import namedtuple
+from unittest import mock
 from unittest.mock import call, MagicMock, patch
 
 import pytest
@@ -78,6 +79,10 @@ def tmpdir_unittest_fixture(request, tmpdir):
 
 @patch("pytorch_lightning.loggers.neptune.neptune", new_callable=create_neptune_mock)
 class TestNeptuneLogger(unittest.TestCase):
+    def run(self, *args, **kwargs):
+        with mock.patch("pytorch_lightning.loggers.neptune._NEPTUNE_AVAILABLE", return_value=True):
+            super().run(*args, **kwargs)
+
     def test_neptune_online(self, neptune):
         logger = NeptuneLogger(api_key="test", project="project")
         created_run_mock = logger.run