fix hparams assign in init

pytorch_lightning/core/lightning.py

@@ -1608,7 +1608,7 @@ def hparams(self) -> Union[AttributeDict, dict, Namespace]:
     @property
     def hparams_initial(self) -> AttributeDict:
         if not hasattr(self, "_hparams_initial"):
-            self._hparams_initial = AttributeDict()
+            return AttributeDict()
         # prevent any change
         return copy.deepcopy(self._hparams_initial)
 
@@ -1617,6 +1617,9 @@ def hparams(self, hp: Union[dict, Namespace, Any]):
         hparams_assignment_name = self.__get_hparams_assignment_variable()
         self._hparams_name = hparams_assignment_name
         self._set_hparams(hp)
+        # this resolves case when user does not uses `save_hyperparameters` and do hard assignement in init
+        if not hasattr(self, "_hparams_initial"):
+            self._hparams_initial = copy.deepcopy(self._hparams)
 
     def __get_hparams_assignment_variable(self):
         """"""

tests/models/test_hparams.py

@@ -556,17 +556,27 @@ def test_args(tmpdir):
         SubClassVarArgs.load_from_checkpoint(raw_checkpoint_path)
 
 
-class RuntimeParamChangeModel(BoringModel):
-    def __init__(self, running_arg):
+class RuntimeParamChangeModelSaving(BoringModel):
+    def __init__(self, **kwargs):
         super().__init__()
         self.save_hyperparameters()
 
 
-def test_init_arg_with_runtime_change(tmpdir):
-    model = RuntimeParamChangeModel(123)
+class RuntimeParamChangeModelAssign(BoringModel):
+    def __init__(self, **kwargs):
+        super().__init__()
+        self.hparams = kwargs
+
+
+@pytest.mark.parametrize("cls", [RuntimeParamChangeModelSaving, RuntimeParamChangeModelAssign])
+def test_init_arg_with_runtime_change(tmpdir, cls):
+    """Test that we save/export only the initial hparams, no other runtime change allowed"""
+    model = cls(running_arg=123)
     assert model.hparams.running_arg == 123
     model.hparams.running_arg = -1
     assert model.hparams.running_arg == -1
+    model.hparams = Namespace(abc=42)
+    assert model.hparams.abc == 42
 
     trainer = Trainer(
         default_root_dir=tmpdir,