bpo-22005: Document the reality of pickle compatibility. (GH-11054)

diff --git a/Doc/library/pickle.rst b/Doc/library/pickle.rst
index 5fe49a013bc4c615561195a64092515e8ff423a3..100a6a134508680c0435ff07ca3aa70434bfaa18 100644 (file)
--- a/Doc/library/pickle.rst
+++ b/Doc/library/pickle.rst
@@ -71,7 +71,9 @@ The :mod:`pickle` module differs from :mod:`marshal` in several significant ways
   :file:`.pyc` files, the Python implementers reserve the right to change the
   serialization format in non-backwards compatible ways should the need arise.
   The :mod:`pickle` serialization format is guaranteed to be backwards compatible
-  across Python releases.
+  across Python releases provided a compatible pickle protocol is chosen and
+  pickling and unpickling code deals with Python 2 to Python 3 type differences
+  if your data is crossing that unique breaking change language boundary.
 
 Comparison with ``json``
 ^^^^^^^^^^^^^^^^^^^^^^^^