bpo-23403: Bump pickle.DEFAULT_PROTOCOL to 4 (#6355)

This makes performance better and produces shorter pickles. This change is backwards compatible up to the oldest currently supported version of Python (3.4).

2 files changed, 17 insertions, 11 deletions

diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 6a684f2..aa672af 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -20,10 +20,12 @@ class _pickle.UnpicklerMemoProxy "UnpicklerMemoProxyObject *" "&UnpicklerMemoPro
 [clinic start generated code]*/
 /*[clinic end generated code: output=da39a3ee5e6b4b0d input=4b3e113468a58e6c]*/
 
-/* Bump this when new opcodes are added to the pickle protocol. */
+/* Bump HIGHEST_PROTOCOL when new opcodes are added to the pickle protocol.
+   Bump DEFAULT_PROTOCOL only when the oldest still supported version of Python
+   already includes it. */
 enum {
     HIGHEST_PROTOCOL = 4,
-    DEFAULT_PROTOCOL = 3
+    DEFAULT_PROTOCOL = 4
 };
 
 /* Pickle opcodes. These must be kept updated with pickle.py.
@@ -7176,8 +7178,9 @@ This is equivalent to ``Pickler(file, protocol).dump(obj)``, but may
 be more efficient.
 
 The optional *protocol* argument tells the pickler to use the given
-protocol supported protocols are 0, 1, 2, 3 and 4.  The default
-protocol is 3; a backward-incompatible protocol designed for Python 3.
+protocol; supported protocols are 0, 1, 2, 3 and 4.  The default
+protocol is 4. It was introduced in Python 3.4, it is incompatible
+with previous versions.
 
 Specifying a negative protocol version selects the highest protocol
 version supported.  The higher the protocol used, the more recent the
@@ -7196,7 +7199,7 @@ to map the new Python 3 names to the old module names used in Python
 static PyObject *
 _pickle_dump_impl(PyObject *module, PyObject *obj, PyObject *file,
                   PyObject *protocol, int fix_imports)
-/*[clinic end generated code: output=a4774d5fde7d34de input=830f8a64cef6f042]*/
+/*[clinic end generated code: output=a4774d5fde7d34de input=93f1408489a87472]*/
 {
     PicklerObject *pickler = _Pickler_New();
 
@@ -7236,7 +7239,8 @@ Return the pickled representation of the object as a bytes object.
 
 The optional *protocol* argument tells the pickler to use the given
 protocol; supported protocols are 0, 1, 2, 3 and 4.  The default
-protocol is 3; a backward-incompatible protocol designed for Python 3.
+protocol is 4. It was introduced in Python 3.4, it is incompatible
+with previous versions.
 
 Specifying a negative protocol version selects the highest protocol
 version supported.  The higher the protocol used, the more recent the
@@ -7250,7 +7254,7 @@ Python 2, so that the pickle data stream is readable with Python 2.
 static PyObject *
 _pickle_dumps_impl(PyObject *module, PyObject *obj, PyObject *protocol,
                    int fix_imports)
-/*[clinic end generated code: output=d75d5cda456fd261 input=293dbeda181580b7]*/
+/*[clinic end generated code: output=d75d5cda456fd261 input=b6efb45a7d19b5ab]*/
 {
     PyObject *result;
     PicklerObject *pickler = _Pickler_New();
diff --git a/Modules/clinic/_pickle.c.h b/Modules/clinic/_pickle.c.h
index 7fc00a0..6d90728 100644
--- a/Modules/clinic/_pickle.c.h
+++ b/Modules/clinic/_pickle.c.h
@@ -367,8 +367,9 @@ PyDoc_STRVAR(_pickle_dump__doc__,
 "be more efficient.\n"
 "\n"
 "The optional *protocol* argument tells the pickler to use the given\n"
-"protocol supported protocols are 0, 1, 2, 3 and 4.  The default\n"
-"protocol is 3; a backward-incompatible protocol designed for Python 3.\n"
+"protocol; supported protocols are 0, 1, 2, 3 and 4.  The default\n"
+"protocol is 4. It was introduced in Python 3.4, it is incompatible\n"
+"with previous versions.\n"
 "\n"
 "Specifying a negative protocol version selects the highest protocol\n"
 "version supported.  The higher the protocol used, the more recent the\n"
@@ -419,7 +420,8 @@ PyDoc_STRVAR(_pickle_dumps__doc__,
 "\n"
 "The optional *protocol* argument tells the pickler to use the given\n"
 "protocol; supported protocols are 0, 1, 2, 3 and 4.  The default\n"
-"protocol is 3; a backward-incompatible protocol designed for Python 3.\n"
+"protocol is 4. It was introduced in Python 3.4, it is incompatible\n"
+"with previous versions.\n"
 "\n"
 "Specifying a negative protocol version selects the highest protocol\n"
 "version supported.  The higher the protocol used, the more recent the\n"
@@ -560,4 +562,4 @@ _pickle_loads(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObjec
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=e995dd494045d876 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=6fc104b8299c82dd input=a9049054013a1b77]*/