pythongh-107089: Improve Shelf.clear method performance

The clear method used to be implemented by inheriting a mix-in from the
MutableMapping ABC. It was a poor fit for shelves, and a better
implementation is now in place

Lib/shelve.py

@@ -171,6 +171,25 @@ def sync(self):
         if hasattr(self.dict, 'sync'):
             self.dict.sync()
 
+    def clear(self):
+        """Remove all items from the shelf."""
+        self.cache.clear()
+        try:
+            self.dict.clear()
+        except AttributeError:
+            # dbm objects don't have a clear method, so we need to do
+            # the clearing here.
+            keys = self.dict.keys()
+            if not isinstance(keys, list):
+                # The keys method on dbm objects returns a list.
+                # Typically, the keys method on a mapping returns a
+                # lazy iterator, so we need to watch out for that in
+                # case someone passes in a backing object that behaves
+                # that way.
+                keys = list(keys)
+            for k in keys:
+                del self.dict[k]
+
 
 class BsdDbShelf(Shelf):
     """Shelf implementation using the "BSD" db interface.

Misc/ACKS

@@ -289,6 +289,7 @@ Edward Catmur
 Lorenzo M. Catucci
 Bruno Cauet
 Donn Cave
+James Cave
 Charles Cazabon
 Jesús Cea Avión
 Per Cederqvist

Misc/NEWS.d/next/Library/2023-07-22-21-57-34.gh-issue-107089.Dnget2.rst

@@ -0,0 +1,2 @@
+The :meth:`Shelf.clear` method in the :mod:`shelve` module is much faster.
+Patch by James Cave.