bpo-34204: Use pickle.DEFAULT_PROTOCOL in shelve

Doc/library/shelve.rst

@@ -25,8 +25,9 @@ lots of shared  sub-objects.  The keys are ordinary strings.
    database file is opened for reading and writing.  The optional *flag* parameter
    has the same interpretation as the *flag* parameter of :func:`dbm.open`.
 
-   By default, version 3 pickles are used to serialize values.  The version of the
-   pickle protocol can be specified with the *protocol* parameter.
+   By default, pickles created with :data:`pickle.DEFAULT_PROTOCOL` are used
+   to serialize values.  The version of the pickle protocol can be specified
+   with the *protocol* parameter.
 
    Because of Python semantics, a shelf cannot know when a mutable
    persistent-dictionary entry is modified.  By default modified objects are
@@ -40,6 +41,11 @@ lots of shared  sub-objects.  The keys are ordinary strings.
    determine which accessed entries are mutable, nor which ones were actually
    mutated).
 
+   .. versionchanged:: 3.9
+
+      :data:`pickle.DEFAULT_PROTOCOL` is now used as the default pickle
+      protocol.
+
    .. note::
 
       Do not rely on the shelf being closed automatically; always call

Lib/shelve.py

@@ -56,7 +56,7 @@
 the persistent dictionary on disk, if feasible).
 """
 
-from pickle import Pickler, Unpickler
+from pickle import DEFAULT_PROTOCOL, Pickler, Unpickler
 from io import BytesIO
 
 import collections.abc
@@ -85,7 +85,7 @@ def __init__(self, dict, protocol=None, writeback=False,
                  keyencoding="utf-8"):
         self.dict = dict
         if protocol is None:
-            protocol = 3
+            protocol = DEFAULT_PROTOCOL
         self._protocol = protocol
         self.writeback = writeback
         self.cache = {}

Lib/test/test_shelve.py

@@ -1,6 +1,8 @@
 import unittest
 import shelve
 import glob
+import pickle
+
 from test import support
 from collections.abc import MutableMapping
 from test.test_dbm import dbm_iterator
@@ -159,7 +161,7 @@ def test_with(self):
 
     def test_default_protocol(self):
         with shelve.Shelf({}) as s:
-            self.assertEqual(s._protocol, 3)
+            self.assertEqual(s._protocol, pickle.DEFAULT_PROTOCOL)
 
 from test import mapping_tests
 

Misc/NEWS.d/next/Library/2020-04-21-17-18-33.bpo-34204.9wXTtY.rst

@@ -0,0 +1,2 @@
+The :mod:`shelve` module now uses :data:`pickle.DEFAULT_PROTOCOL` by default
+instead of :mod:`pickle` protocol ``3``.