Fix bugs arising from None and empty dicts causing dupes.

scone/head/dag.py

@@ -92,13 +92,32 @@ class Resource:
     though should only be used where necessary and sensible to do so.
     """
     # extra_params: Optional[frozendict[str, str]] = None
-    extra_params: Optional[frozendict] = None
+    extra_params: Optional[frozendict] = attr.ib(default=None)
 
     def __str__(self) -> str:
-        extra_str = "" if not self.extra_params else f" {self.extra_params!r}"
+        extra_str = "" if not self.extra_params else f"{self.extra_params!r}"
         sous_str = "" if not self.sous else f" on {self.sous}"
         return f"{self.kind}({self.id}){extra_str}{sous_str}"
 
+    @extra_params.validator
+    def _check_extra_params(self, _attribute, value):
+        if value is not None and len(value) == 0:
+            raise ValueError(
+                "Resources must not contain an empty extra_params dict."
+                " Use None instead."
+            )
+
+    @staticmethod
+    def new_lenient(
+        kind: str, id: str, sous: Optional[str], extra_params: Optional[frozendict]
+    ):
+        """
+        Alternative constructor which will correct an empty extra_params dict.
+        """
+        if extra_params is not None and len(extra_params) == 0:
+            extra_params = None
+        return Resource(kind, id, sous, extra_params)
+
 
 @attr.s(auto_attribs=True)
 class ResourceMeta:

scone/head/kitchen.py

@@ -70,7 +70,7 @@ class Preparation:
         if sous == "(self)":
             sous = self._current_recipe.recipe_context.sous
 
-        resource = Resource(
+        resource = Resource.new_lenient(
             requirement, identifier, sous, frozendict(extra_identifiers)
         )
 
@@ -91,7 +91,7 @@ class Preparation:
         if sous == "(self)":
             sous = self._current_recipe.recipe_context.sous
 
-        resource = Resource(
+        resource = Resource.new_lenient(
             requirement, identifier, sous, frozendict(extra_identifiers)
         )