diff --git a/swh/loader/core/utils.py b/swh/loader/core/utils.py
index 89684f2..de026c2 100644
--- a/swh/loader/core/utils.py
+++ b/swh/loader/core/utils.py
@@ -1,48 +1,45 @@
-# Copyright (C) 2018  The Software Heritage developers
+# Copyright (C) 2018-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 
 import os
 import shutil
 
 import psutil
 
 
-def clean_dangling_folders(dirpath, pattern_check, log=None):
-    """Clean up potential dangling temporary working folder rooted at
-       `dirpath`. Those folders must match a dedicated pattern and not
-       belonging to a live pid.
+def clean_dangling_folders(dirpath: str, pattern_check: str, log=None) -> None:
+    """Clean up potential dangling temporary working folder rooted at `dirpath`. Those
+       folders must match a dedicated pattern and not belonging to a live pid.
 
     Args:
-        dirpath (str): Path to check for dangling files
-        pattern_check (str): A dedicated pattern to check on first
-            level directory (e.g `swh.loader.mercurial.`,
-            `swh.loader.svn.`)
-            log (Logger): Optional logger
+        dirpath: Path to check for dangling files
+        pattern_check: A dedicated pattern to check on first level directory (e.g
+            `swh.loader.mercurial.`, `swh.loader.svn.`)
+        log (Logger): Optional logger
 
     """
     if not os.path.exists(dirpath):
         return
     for filename in os.listdir(dirpath):
         path_to_cleanup = os.path.join(dirpath, filename)
         try:
-            # pattern: `swh.loader.svn-pid.{noise}`
+            # pattern: `swh.loader.{loader-type}-pid.{noise}`
             if (
                 pattern_check not in filename or "-" not in filename
             ):  # silently ignore unknown patterns
                 continue
-            _, pid = filename.split("-")
-            pid = int(pid.split(".")[0])
+            _, pid_ = filename.split("-")
+            pid = int(pid_.split(".")[0])
             if psutil.pid_exists(pid):
                 if log:
-                    log.debug("PID %s is live, skipping" % pid)
+                    log.debug("PID %s is live, skipping", pid)
                 continue
             # could be removed concurrently, so check before removal
             if os.path.exists(path_to_cleanup):
                 shutil.rmtree(path_to_cleanup)
         except Exception as e:
             if log:
-                msg = "Fail to clean dangling path %s: %s" % (path_to_cleanup, e)
-                log.warn(msg)
+                log.warn("Fail to clean dangling path %s: %s", path_to_cleanup, e)