diff --git a/mercurial/lock.py b/mercurial/lock.py
--- a/mercurial/lock.py
+++ b/mercurial/lock.py
@@ -204,7 +204,7 @@
 
     def __del__(self):
         if self.held:
-            warnings.warn("use lock.release instead of del lock",
+            warnings.warn(r"use lock.release instead of del lock",
                     category=DeprecationWarning,
                     stacklevel=2)