[Python-checkins] bpo-22005: Document the reality of pickle compatibility. (GH-11054)

[Miss Islington (bot)]

https://github.com/python/cpython/commit/331bfa4f2c3026a35e111303df0f198d06b4e0c8
commit: 331bfa4f2c3026a35e111303df0f198d06b4e0c8
branch: 3.7
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2018-12-09T11:48:35-08:00
summary:

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

(cherry picked from commit e328753d91379274b699b93decff45de07854617)

Co-authored-by: Gregory P. Smith <greg at krypto.org>

files:
M Doc/library/pickle.rst

diff --git a/Doc/library/pickle.rst b/Doc/library/pickle.rst
index 52cbb6241bc9..36f66a159bf2 100644
--- 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``
 ^^^^^^^^^^^^^^^^^^^^^^^^