diff --git a/swh/loader/mercurial/loader.py b/swh/loader/mercurial/loader.py
--- a/swh/loader/mercurial/loader.py
+++ b/swh/loader/mercurial/loader.py
@@ -162,7 +162,7 @@
 
         return b
 
-    def prepare_origin_visit(self, *args, **kwargs):
+    def prepare_origin_visit(self, *args, **kwargs) -> None:
         self.origin = Origin(url=self.origin_url)
         visit_date = self.visit_date
         if isinstance(visit_date, str):  # visit_date can be string or datetime
@@ -357,7 +357,7 @@
         if contents:
             missing_contents = set(
                 self.storage.content_missing(
-                    map(lambda c: c.to_dict(), contents.values()), key_hash=ALGO
+                    [c.to_dict() for c in contents.values()], key_hash=ALGO
                 )
             )
 
@@ -516,7 +516,7 @@
 
         missing_revs = set(revisions.keys())
         if missing_revs:
-            missing_revs = set(self.storage.revision_missing(missing_revs))
+            missing_revs = set(self.storage.revision_missing(list(missing_revs)))
 
         for rev in missing_revs:
             yield revisions[rev]
@@ -562,7 +562,7 @@
             self.releases[name] = release.id
 
         if missing_releases:
-            missing_releases = set(self.storage.release_missing(missing_releases))
+            missing_releases = set(self.storage.release_missing(list(missing_releases)))
 
         for _id in missing_releases:
             yield releases[_id]