diff --git a/swh/lister/maven/lister.py b/swh/lister/maven/lister.py
--- a/swh/lister/maven/lister.py
+++ b/swh/lister/maven/lister.py
@@ -141,7 +141,7 @@
         # ]
 
         # Download the main text index file.
-        logger.info(f"Downloading text index from {self.INDEX_URL}.")
+        logger.info("Downloading text index from %s.", self.INDEX_URL)
         assert self.INDEX_URL is not None
         response = requests.get(self.INDEX_URL, stream=True)
         response.raise_for_status()
@@ -229,15 +229,18 @@
                     if m_time is not None and url_src is not None:
                         time = m_time.group("mtime")
                         jar_src["time"] = int(time)
-                        logger.debug(f"* Yielding jar {url_src}.")
-                        yield {
+                        artifact_metadata_d = {
                             "type": "maven",
                             "url": url_src,
                             **jar_src,
                         }
+                        logger.debug(
+                            "* Yielding jar %s: %s", url_src, artifact_metadata_d
+                        )
+                        yield artifact_metadata_d
                         url_src = None
 
-        logger.info(f"Found {len(out_pom)} poms.")
+        logger.info("Found %s poms.", len(out_pom))
 
         # Now fetch pom files and scan them for scm info.
 
@@ -251,18 +254,20 @@
                         scm = project["project"]["scm"]["connection"]
                         gid = project["project"]["groupId"]
                         aid = project["project"]["artifactId"]
-                        yield {
+                        artifact_metadata_d = {
                             "type": "scm",
                             "doc": out_pom[pom],
                             "url": scm,
                             "project": f"{gid}.{aid}",
                         }
+                        logger.debug("* Yielding pom %s: %s", pom, artifact_metadata_d)
+                        yield artifact_metadata_d
                     else:
-                        logger.debug(f"No scm.connection in pom {pom}")
+                        logger.debug("No scm.connection in pom %s", pom)
                 else:
-                    logger.debug(f"No scm in pom {pom}")
+                    logger.debug("No scm in pom %s", pom)
             except xmltodict.expat.ExpatError as error:
-                logger.info(f"Could not parse POM {pom} XML: {error}. Next.")
+                logger.info("Could not parse POM %s XML: %s. Next.", pom, error)
 
     def get_origins_from_page(self, page: RepoPage) -> Iterator[ListedOrigin]:
         """Convert a page of Maven repositories into a list of ListedOrigins.