bpo-30550: Clarify JSON ordering guarantees (GH-15397) (GH-15403)

(cherry picked from commit 657008ea0336ff4f275ed3f0c2b6dd2e52de2bba)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>

diff --git a/Doc/library/json.rst b/Doc/library/json.rst
index b476c372370d067f8bc412d4cfd4dc847b3e7a0f..56c16f5d296a3b0cd38a6db084461268359312f4 100644 (file)
--- a/Doc/library/json.rst
+++ b/Doc/library/json.rst
@@ -120,6 +120,18 @@ See :ref:`json-commandline` for detailed documentation.
    value) is also a subset of YAML 1.0 and 1.1.  This module can thus also be
    used as a YAML serializer.
 
+.. note::
+
+   This module's encoders and decoders preserve input and output order by
+   default.  Order is only lost if the underlying containers are unordered.
+
+   Prior to Python 3.7, :class:`dict` was not guaranteed to be ordered, so
+   inputs and outputs were typically scrambled unless
+   :class:`collections.OrderedDict` was specifically requested.  Starting
+   with Python 3.7, the regular :class:`dict` became order preserving, so
+   it is no longer necessary specify :class:`collections.OrderedDict` for
+   JSON generation and parsing.
+
 
 Basic Usage
 -----------