add global SENTINEL object

gallery_dl/config.py

@@ -139,7 +139,6 @@ def unset(path, key, *, conf=_config):
 
 class apply():
     """Context Manager: apply a collection of key-value pairs"""
-    _sentinel = object()
 
     def __init__(self, kvlist):
         self.original = []
@@ -147,12 +146,12 @@ class apply():
 
     def __enter__(self):
         for path, key, value in self.kvlist:
-            self.original.append((path, key, get(path, key, self._sentinel)))
+            self.original.append((path, key, get(path, key, util.SENTINEL)))
             set(path, key, value)
 
     def __exit__(self, etype, value, traceback):
         for path, key, value in self.original:
-            if value is self._sentinel:
+            if value is util.SENTINEL:
                 unset(path, key)
             else:
                 set(path, key, value)