diff --git a/swh/loader/package/cran/loader.py b/swh/loader/package/cran/loader.py
--- a/swh/loader/package/cran/loader.py
+++ b/swh/loader/package/cran/loader.py
@@ -88,7 +88,8 @@
             if version == p_info.version:
                 yield release_name(version), p_info
 
-    def known_artifact_to_extid(self, known_artifact: Dict) -> Optional[PartialExtID]:
+    @staticmethod
+    def known_artifact_to_extid(known_artifact: Dict) -> Optional[PartialExtID]:
         return CRANPackageInfo.from_metadata(known_artifact["extrinsic"]["raw"]).extid()
 
     def build_revision(
diff --git a/swh/loader/package/nixguix/loader.py b/swh/loader/package/nixguix/loader.py
--- a/swh/loader/package/nixguix/loader.py
+++ b/swh/loader/package/nixguix/loader.py
@@ -155,7 +155,8 @@
             ret[revision.id] = revision.metadata
         return ret
 
-    def known_artifact_to_extid(self, known_artifact: Dict) -> Optional[PartialExtID]:
+    @staticmethod
+    def known_artifact_to_extid(known_artifact: Dict) -> Optional[PartialExtID]:
         try:
             value = known_artifact["extrinsic"]["raw"]["integrity"].encode("ascii")
         except KeyError as e: