bpo-37210: Fix pure Python pickle when _pickle is unavailable (GH-14016)

Allow pure Python implementation of pickle to work
even when the C _pickle module is unavailable.

Fix test_pickle when _pickle is missing: declare PyPicklerHookTests
outside "if has_c_implementation:" block.

Lib/pickle.py

@@ -36,10 +36,16 @@ import io
 import codecs
 import _compat_pickle
 
-from _pickle import PickleBuffer
-
 __all__ = ["PickleError", "PicklingError", "UnpicklingError", "Pickler",
-           "Unpickler", "dump", "dumps", "load", "loads", "PickleBuffer"]
+           "Unpickler", "dump", "dumps", "load", "loads"]
+
+try:
+    from _pickle import PickleBuffer
+    __all__.append("PickleBuffer")
+    _HAVE_PICKLE_BUFFER = True
+except ImportError:
+    _HAVE_PICKLE_BUFFER = False
+
 
 # Shortcut for use in isinstance testing
 bytes_types = (bytes, bytearray)
@@ -812,6 +818,7 @@ class _Pickler:
             self.write(BYTEARRAY8 + pack("<Q", n) + obj)
     dispatch[bytearray] = save_bytearray
 
+    if _HAVE_PICKLE_BUFFER:
         def save_picklebuffer(self, obj):
             if self.proto < 5:
                 raise PicklingError("PickleBuffer can only pickled with "

Lib/test/test_pickle.py

@@ -203,6 +203,13 @@ class PyChainDispatchTableTests(AbstractDispatchTableTests):
         return collections.ChainMap({}, pickle.dispatch_table)
 
 
+class PyPicklerHookTests(AbstractHookTests):
+    class CustomPyPicklerClass(pickle._Pickler,
+                               AbstractCustomPicklerClass):
+        pass
+    pickler_class = CustomPyPicklerClass
+
+
 if has_c_implementation:
     class CPickleTests(AbstractPickleModuleTests):
         from _pickle import dump, dumps, load, loads, Pickler, Unpickler
@@ -255,12 +262,6 @@ if has_c_implementation:
         def get_dispatch_table(self):
             return collections.ChainMap({}, pickle.dispatch_table)
 
-    class PyPicklerHookTests(AbstractHookTests):
-        class CustomPyPicklerClass(pickle._Pickler,
-                                   AbstractCustomPicklerClass):
-            pass
-        pickler_class = CustomPyPicklerClass
-
     class CPicklerHookTests(AbstractHookTests):
         class CustomCPicklerClass(_pickle.Pickler, AbstractCustomPicklerClass):
             pass

Misc/NEWS.d/next/Library/2019-06-12-16-10-50.bpo-37210.r4yMg6.rst

@@ -0,0 +1 @@
+Allow pure Python implementation of :mod:`pickle` to work even when the C :mod:`_pickle` module is unavailable.