Enable initialization with "empty" datamodel

src/roman_datamodels/datamodels/_core.py

@@ -90,18 +90,22 @@ def __init__(self, init=None, **kwargs):
                 return
 
         if init is None:
-            self._asdf = self.open_asdf(init=None, **kwargs)
+            self._instance = self._node_type()
+
         elif isinstance(init, (str, bytes, PurePath)):
             if isinstance(init, PurePath):
                 init = str(init)
             if isinstance(init, bytes):
                 init = init.decode(sys.getfilesystemencoding())
+
             self._asdf = self.open_asdf(init, **kwargs)
             if not self.check_type(self._asdf):
                 raise ValueError(f"ASDF file is not of the type expected. Expected {self.__class__.__name__}")
+
             self._instance = self._asdf.tree["roman"]
         elif isinstance(init, asdf.AsdfFile):
             self._asdf = init
+
             self._instance = self._asdf.tree["roman"]
         else:
             raise OSError("Argument does not appear to be an ASDF file or TaggedObjectNode.")
@@ -303,11 +307,18 @@ def validate(self):
         """
         validate.value_change(self._instance, pass_invalid_values=False, strict_validation=True)
 
+    @property
+    def asdf(self):
+        if self._asdf is None:
+            self._asdf = asdf.AsdfFile({"roman": self._instance})
+
+        return self._asdf
+
     def info(self, *args, **kwargs):
-        return self._asdf.info(*args, **kwargs)
+        return self.asdf.info(*args, **kwargs)
 
     def search(self, *args, **kwargs):
-        return self._asdf.search(*args, **kwargs)
+        return self.asdf.search(*args, **kwargs)
 
     def schema_info(self, *args, **kwargs):
-        return self._asdf.schema_info(*args, **kwargs)
+        return self.asdf.schema_info(*args, **kwargs)

tests/test_models.py

@@ -63,6 +63,29 @@ def test_model_schemas(node, model):
     asdf.schema.load_schema(instance.schema_uri)
 
 
+@pytest.mark.parametrize("node, model", datamodels.MODEL_REGISTRY.items())
+def test_empty_model(node, model):
+    mdl = model()
+    assert isinstance(mdl._instance, node)
+    assert mdl._asdf is None
+
+
+@pytest.mark.parametrize("node, model", datamodels.MODEL_REGISTRY.items())
+def test_empty_model_asdf(node, model):
+    mdl = model()
+
+    # Check there is a validation issue
+    with pytest.raises(asdf.ValidationError):
+        mdl.asdf
+
+    assert mdl._asdf is None
+
+    # Fill in instance properly and check that asdf can be created
+    mdl._instance = utils.mk_node(node)
+    assert mdl.asdf.tree["roman"] is mdl._instance
+    assert mdl._asdf is not None
+
+
 # Testing core schema
 def test_core_schema(tmp_path):
     # Set temporary asdf file