Add validation of top_level class attribute, fix a bug in str() output

Author: glennmatthews

diffsync/__init__.py

@@ -428,6 +428,13 @@ def __init_subclass__(cls):
                     f'Incorrect field name - {value.__name__} has type name "{value.get_type()}", not "{name}"'
                 )
 
+        for name in cls.top_level:
+            if not hasattr(cls, name):
+                raise AttributeError(f'top_level references attribute "{name}" but it is not a class attribute!')
+            value = getattr(cls, name)
+            if not isclass(value) or not issubclass(value, DiffSyncModel):
+                raise AttributeError(f'top_level references attribute "{name}" but it is not a DiffSyncModel subclass!')
+
     def __str__(self):
         """String representation of a DiffSync."""
         if self.type != self.name:
@@ -455,6 +462,8 @@ def str(self, indent: int = 0) -> str:
         margin = " " * indent
         output = ""
         for modelname in self.top_level:
+            if output:
+                output += "\n"
             output += f"{margin}{modelname}"
             models = self.get_all(modelname)
             if not models:

tests/unit/conftest.py

@@ -140,14 +140,24 @@ def generic_diffsync():
     return DiffSync()
 
 
+class UnusedModel(DiffSyncModel):
+    """Concrete DiffSyncModel subclass that can be referenced as a class attribute but never has any data."""
+
+    _modelname = "unused"
+    _identifiers = ("name",)
+
+    name: str
+
+
 class GenericBackend(DiffSync):
     """An example semi-abstract subclass of DiffSync."""
 
     site = Site  # to be overridden by subclasses
     device = Device
     interface = Interface
+    unused = UnusedModel
 
-    top_level = ["site"]
+    top_level = ["site", "unused"]
 
     DATA: dict = {}
 

tests/unit/test_diffsync.py

@@ -132,13 +132,12 @@ def test_diffsync_remove_with_generic_model(generic_diffsync, generic_diffsync_m
         generic_diffsync.get_by_uids([""], DiffSyncModel)
 
 
-def test_diffsync_subclass_validation():
-    """Test the declaration-time checks on a DiffSync subclass."""
+def test_diffsync_subclass_validation_name_mismatch():
     # pylint: disable=unused-variable
     with pytest.raises(AttributeError) as excinfo:
 
         class BadElementName(DiffSync):
-            """Model with a DiffSyncModel attribute whose name does not match the modelname."""
+            """DiffSync with a DiffSyncModel attribute whose name does not match the modelname."""
 
             dev_class = Device  # should be device = Device
 
@@ -147,6 +146,33 @@ class BadElementName(DiffSync):
     assert "dev_class" in str(excinfo.value)
 
 
+def test_diffsync_subclass_validation_missing_top_level():
+    with pytest.raises(AttributeError) as excinfo:
+
+        class MissingTopLevel(DiffSync):
+            """DiffSync whose top_level references an attribute that does not exist on the class."""
+
+            top_level = ["missing"]
+
+    assert "top_level" in str(excinfo.value)
+    assert "missing" in str(excinfo.value)
+    assert "is not a class attribute" in str(excinfo.value)
+
+
+def test_diffsync_subclass_validation_top_level_not_diffsyncmodel():
+    with pytest.raises(AttributeError) as excinfo:
+
+        class TopLevelNotDiffSyncModel(DiffSync):
+            """DiffSync whose top_level references an attribute that is not a DiffSyncModel subclass."""
+
+            age = 0
+            top_level = ["age"]
+
+    assert "top_level" in str(excinfo.value)
+    assert "age" in str(excinfo.value)
+    assert "is not a DiffSyncModel" in str(excinfo.value)
+
+
 def test_diffsync_dict_with_data(backend_a):
     assert backend_a.dict() == {
         "device": {
@@ -250,7 +276,9 @@ def test_diffsync_str_with_data(backend_a):
           interface: rdu-spine2__eth0: {'interface_type': 'ethernet', 'description': 'Interface 0'}
           interface: rdu-spine2__eth1: {'interface_type': 'ethernet', 'description': 'Interface 1'}
     people
-      person: Glenn Matthews: {}"""
+      person: Glenn Matthews: {}
+unused: []\
+"""
     )