pythongh-107089: call dbm.clear method

Because pythongh-107089 is peculiar to implementation details of dbm objects,
it would be less disruptive to implement it in the DbfilenameShelf
class, which is used for calls to shelve.open. Since it is known that
the backing object is specifically one of the dbm objects, its clear
method (see pythongh-107122) can be used with no fallback code.

Lib/shelve.py

@@ -171,26 +171,6 @@ 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.
 
@@ -244,6 +224,13 @@ class DbfilenameShelf(Shelf):
     def __init__(self, filename, flag='c', protocol=None, writeback=False):
         import dbm
         Shelf.__init__(self, dbm.open(filename, flag), protocol, writeback)
+
+    def clear(self):
+        """Remove all items from the shelf."""
+        # Call through to the clear method on dbm-backed shelves.
+        # see https://github.com/python/cpython/issues/107089
+        self.cache.clear()
+        self.dict.clear()
 
 
 def open(filename, flag='c', protocol=None, writeback=False):

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

@@ -1,2 +1,2 @@
-The :meth:`Shelf.clear` method in the :mod:`shelve` module is much faster.
-Patch by James Cave.
+Shelves opened with :func:`shelve.open` have a much faster :meth:`clear`
+method. Patch by James Cave.