Issue #14674: Add a discussion of the json module's standard compliance.

Patch by Chris Rebert.

Doc/library/json.rst

@@ -7,8 +7,10 @@
 .. sectionauthor:: Bob Ippolito <bob@redivi.com>
 .. versionadded:: 2.6
 
-`JSON (JavaScript Object Notation) <http://json.org>`_ is a subset of JavaScript
-syntax (ECMA-262 3rd edition) used as a lightweight data interchange format.
+`JSON (JavaScript Object Notation) <http://json.org>`_, specified by
+:rfc:`4627`, is a lightweight data interchange format based on a subset of
+`JavaScript <http://en.wikipedia.org/wiki/JavaScript>`_ syntax (`ECMA-262 3rd
+edition <http://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%203rd%20edition,%20December%201999.pdf>`_).
 
 :mod:`json` exposes an API familiar to users of the standard library
 :mod:`marshal` and :mod:`pickle` modules.
@@ -106,8 +108,10 @@ Using json.tool from the shell to validate and pretty-print::
 
 .. note::
 
-   The JSON produced by this module's default settings is a subset of
-   YAML, so it may be used as a serializer for that as well.
+   JSON is a subset of `YAML <http://yaml.org/>`_ 1.2.  The JSON produced by
+   this module's default settings (in particular, the default *separators*
+   value) is also a subset of YAML 1.0 and 1.1.  This module can thus also be
+   used as a YAML serializer.
 
 
 Basic Usage
@@ -193,7 +197,8 @@ Basic Usage
    *object_hook* is an optional function that will be called with the result of
    any object literal decoded (a :class:`dict`).  The return value of
    *object_hook* will be used instead of the :class:`dict`.  This feature can be used
-   to implement custom decoders (e.g. JSON-RPC class hinting).
+   to implement custom decoders (e.g. `JSON-RPC <http://www.jsonrpc.org>`_
+   class hinting).
 
    *object_pairs_hook* is an optional function that will be called with the
    result of any object literal decoded with an ordered list of pairs.  The
@@ -242,7 +247,7 @@ Basic Usage
    The other arguments have the same meaning as in :func:`load`.
 
 
-Encoders and decoders
+Encoders and Decoders
 ---------------------
 
 .. class:: JSONDecoder([encoding[, object_hook[, parse_float[, parse_int[, parse_constant[, strict[, object_pairs_hook]]]]]]])
@@ -438,3 +443,108 @@ Encoders and decoders
 
             for chunk in JSONEncoder().iterencode(bigobject):
                 mysocket.write(chunk)
+
+
+Standard Compliance
+-------------------
+
+The JSON format is specified by :rfc:`4627`.  This section details this
+module's level of compliance with the RFC.  For simplicity,
+:class:`JSONEncoder` and :class:`JSONDecoder` subclasses, and parameters other
+than those explicitly mentioned, are not considered.
+
+This module does not comply with the RFC in a strict fashion, implementing some
+extensions that are valid JavaScript but not valid JSON.  In particular:
+
+- Top-level non-object, non-array values are accepted and output;
+- Infinite and NaN number values are accepted and output;
+- Repeated names within an object are accepted, and only the value of the last
+  name-value pair is used.
+
+Since the RFC permits RFC-compliant parsers to accept input texts that are not
+RFC-compliant, this module's deserializer is technically RFC-compliant under
+default settings.
+
+Character Encodings
+^^^^^^^^^^^^^^^^^^^
+
+The RFC recommends that JSON be represented using either UTF-8, UTF-16, or
+UTF-32, with UTF-8 being the default.  Accordingly, this module uses UTF-8 as
+the default for its *encoding* parameter.
+
+This module's deserializer only directly works with ASCII-compatible encodings;
+UTF-16, UTF-32, and other ASCII-incompatible encodings require the use of
+workarounds described in the documentation for the deserializer's *encoding*
+parameter.
+
+The RFC also non-normatively describes a limited encoding detection technique
+for JSON texts; this module's deserializer does not implement this or any other
+kind of encoding detection.
+
+As permitted, though not required, by the RFC, this module's serializer sets
+*ensure_ascii=True* by default, thus escaping the output so that the resulting
+strings only contain ASCII characters.
+
+
+Top-level Non-Object, Non-Array Values
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The RFC specifies that the top-level value of a JSON text must be either a
+JSON object or array (Python :class:`dict` or :class:`list`).  This module's
+deserializer also accepts input texts consisting solely of a
+JSON null, boolean, number, or string value::
+
+   >>> just_a_json_string = '"spam and eggs"'  # Not by itself a valid JSON text
+   >>> json.loads(just_a_json_string)
+   u'spam and eggs'
+
+This module itself does not include a way to request that such input texts be
+regarded as illegal.  Likewise, this module's serializer also accepts single
+Python :data:`None`, :class:`bool`, numeric, and :class:`str`
+values as input and will generate output texts consisting solely of a top-level
+JSON null, boolean, number, or string value without raising an exception::
+
+   >>> neither_a_list_nor_a_dict = u"spam and eggs"
+   >>> json.dumps(neither_a_list_nor_a_dict)  # The result is not a valid JSON text
+   '"spam and eggs"'
+
+This module's serializer does not itself include a way to enforce the
+aforementioned constraint.
+
+
+Infinite and NaN Number Values
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The RFC does not permit the representation of infinite or NaN number values.
+Despite that, by default, this module accepts and outputs ``Infinity``,
+``-Infinity``, and ``NaN`` as if they were valid JSON number literal values::
+
+   >>> # Neither of these calls raises an exception, but the results are not valid JSON
+   >>> json.dumps(float('-inf'))
+   '-Infinity'
+   >>> json.dumps(float('nan'))
+   'NaN'
+   >>> # Same when deserializing
+   >>> json.loads('-Infinity')
+   -inf
+   >>> json.loads('NaN')
+   nan
+
+In the serializer, the *allow_nan* parameter can be used to alter this
+behavior.  In the deserializer, the *parse_constant* parameter can be used to
+alter this behavior.
+
+
+Repeated Names Within an Object
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The RFC specifies that the names within a JSON object should be unique, but
+does not specify how repeated names in JSON objects should be handled.  By
+default, this module does not raise an exception; instead, it ignores all but
+the last name-value pair for a given name::
+
+   >>> weird_json = '{"x": 1, "x": 2, "x": 3}'
+   >>> json.loads(weird_json)
+   {u'x': 3}
+
+The *object_pairs_hook* parameter can be used to alter this behavior.

Misc/NEWS

@@ -373,6 +373,9 @@ Build
 Documentation
 -------------
 
+- Issue #14674: Add a discussion of the json module's standard compliance.
+  Patch by Chris Rebert.
+
 - Issue #15630: Add an example for "continue" stmt in the tutorial. Patch by
   Daniel Ellis.