bpo-34204: Use pickle.DEFAULT_PROTOCOL in shelve (GH-19639)

Use pickle.DEFAULT_PROTOCOL (currently 5) in shelve instead of a
hardcoded 3.

1 files changed, 2 insertions, 2 deletions

diff --git a/Lib/shelve.py b/Lib/shelve.py
index 5d443a0fa8..e053c39734 100644
--- a/Lib/shelve.py
+++ b/Lib/shelve.py
@@ -56,7 +56,7 @@ entries in the cache, and empty the cache (d.sync() also synchronizes
 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 @@ class Shelf(collections.abc.MutableMapping):
                  keyencoding="utf-8"):
         self.dict = dict
         if protocol is None:
-            protocol = 3
+            protocol = DEFAULT_PROTOCOL
         self._protocol = protocol
         self.writeback = writeback
         self.cache = {}