Add clarification on why a set is used in get_not_required

Author: pip

src/pip/_internal/commands/list.py

@@ -190,9 +190,14 @@ def get_not_required(self, packages, options):
         dep_keys = set()  # type: Set[Distribution]
         for dist in packages:
             dep_keys.update(requirement.key for requirement in dist.requires())
+
+        # Create a set to remove duplicate packages, and cast it to a list
+        # to keep the return type consistent with get_outdated and
+        # get_uptodate
         return list({pkg for pkg in packages if pkg.key not in dep_keys})
 
-    def iter_packages_latest_infos(self, packages, options):  # type: ignore
+    def iter_packages_latest_infos(self, packages, options):
+        # type: (List[Distribution], Values) -> Distribution
         with self._build_session(options) as session:
             finder = self._build_package_finder(options, session)