Convert error to warning for logging without a Trainer

CHANGELOG.md

@@ -206,6 +206,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Improve coverage of `self.log`-ing in any `LightningModule` or `Callback` hook ([#8498](https://github.com/PyTorchLightning/pytorch-lightning/pull/8498))
 
 
+- `self.log`-ing without a `Trainer` reference now raises a warning instead of an exception ([#9733](https://github.com/PyTorchLightning/pytorch-lightning/pull/9733))
+
+
 - Removed restrictions in the trainer that loggers can only log from rank 0. Existing logger behavior has not changed. ([#8608](https://github.com/PyTorchLightning/pytorch-lightning/pull/8608))
 
 

pytorch_lightning/core/lightning.py

@@ -391,10 +391,12 @@ def log(
         on_epoch = self.__auto_choose_log_on_epoch(on_epoch)
 
         if self.trainer is None:
-            raise MisconfigurationException(
+            # not an error to support testing the `*_step` methods without a `Trainer` reference
+            rank_zero_warn(
                 "You are trying to `self.log()` but the `self.trainer` reference is not registered on the model yet."
                 " This is most likely because the model hasn't been passed to the `Trainer`"
             )
+            return
         results = self.trainer._results
         if results is None:
             raise MisconfigurationException(

tests/trainer/logging_/test_logger_connector.py

@@ -237,7 +237,7 @@ def test_fx_validator_integration(tmpdir):
     }
     model = HookedModel(not_supported)
 
-    with pytest.raises(MisconfigurationException, match=not_supported[None]):
+    with pytest.warns(UserWarning, match=not_supported[None]):
         model.log("foo", 1)
 
     callback = HookedCallback(not_supported)