Issue #14674: Add a discussion of the json module's standard compliance.
Patch by Chris Rebert.
This commit is contained in:
Antoine Pitrou
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.. sectionauthor:: Bob Ippolito <bob@redivi.com>
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.. versionadded:: 2.6
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`JSON (JavaScript Object Notation) <http://json.org>`_ is a subset of JavaScript
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syntax (ECMA-262 3rd edition) used as a lightweight data interchange format.
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`JSON (JavaScript Object Notation) <http://json.org>`_, specified by
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:rfc:`4627`, is a lightweight data interchange format based on a subset of
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`JavaScript <http://en.wikipedia.org/wiki/JavaScript>`_ syntax (`ECMA-262 3rd
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edition <http://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%203rd%20edition,%20December%201999.pdf>`_).
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:mod:`json` exposes an API familiar to users of the standard library
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:mod:`marshal` and :mod:`pickle` modules.
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@ -106,8 +108,10 @@ Using json.tool from the shell to validate and pretty-print::
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.. note::
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The JSON produced by this module's default settings is a subset of
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YAML, so it may be used as a serializer for that as well.
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JSON is a subset of `YAML <http://yaml.org/>`_ 1.2. The JSON produced by
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this module's default settings (in particular, the default *separators*
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value) is also a subset of YAML 1.0 and 1.1. This module can thus also be
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used as a YAML serializer.
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Basic Usage
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@ -193,7 +197,8 @@ Basic Usage
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*object_hook* is an optional function that will be called with the result of
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any object literal decoded (a :class:`dict`). The return value of
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*object_hook* will be used instead of the :class:`dict`. This feature can be used
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to implement custom decoders (e.g. JSON-RPC class hinting).
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to implement custom decoders (e.g. `JSON-RPC <http://www.jsonrpc.org>`_
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class hinting).
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*object_pairs_hook* is an optional function that will be called with the
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result of any object literal decoded with an ordered list of pairs. The
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@ -242,7 +247,7 @@ Basic Usage
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The other arguments have the same meaning as in :func:`load`.
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Encoders and decoders
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Encoders and Decoders
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---------------------
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.. class:: JSONDecoder([encoding[, object_hook[, parse_float[, parse_int[, parse_constant[, strict[, object_pairs_hook]]]]]]])
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@ -438,3 +443,108 @@ Encoders and decoders
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for chunk in JSONEncoder().iterencode(bigobject):
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mysocket.write(chunk)
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Standard Compliance
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-------------------
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The JSON format is specified by :rfc:`4627`. This section details this
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module's level of compliance with the RFC. For simplicity,
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:class:`JSONEncoder` and :class:`JSONDecoder` subclasses, and parameters other
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than those explicitly mentioned, are not considered.
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This module does not comply with the RFC in a strict fashion, implementing some
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extensions that are valid JavaScript but not valid JSON. In particular:
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- Top-level non-object, non-array values are accepted and output;
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- Infinite and NaN number values are accepted and output;
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- Repeated names within an object are accepted, and only the value of the last
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name-value pair is used.
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Since the RFC permits RFC-compliant parsers to accept input texts that are not
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RFC-compliant, this module's deserializer is technically RFC-compliant under
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default settings.
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Character Encodings
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^^^^^^^^^^^^^^^^^^^
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The RFC recommends that JSON be represented using either UTF-8, UTF-16, or
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UTF-32, with UTF-8 being the default. Accordingly, this module uses UTF-8 as
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the default for its *encoding* parameter.
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This module's deserializer only directly works with ASCII-compatible encodings;
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UTF-16, UTF-32, and other ASCII-incompatible encodings require the use of
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workarounds described in the documentation for the deserializer's *encoding*
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parameter.
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The RFC also non-normatively describes a limited encoding detection technique
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for JSON texts; this module's deserializer does not implement this or any other
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kind of encoding detection.
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As permitted, though not required, by the RFC, this module's serializer sets
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*ensure_ascii=True* by default, thus escaping the output so that the resulting
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strings only contain ASCII characters.
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Top-level Non-Object, Non-Array Values
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The RFC specifies that the top-level value of a JSON text must be either a
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JSON object or array (Python :class:`dict` or :class:`list`). This module's
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deserializer also accepts input texts consisting solely of a
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JSON null, boolean, number, or string value::
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>>> just_a_json_string = '"spam and eggs"' # Not by itself a valid JSON text
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>>> json.loads(just_a_json_string)
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u'spam and eggs'
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This module itself does not include a way to request that such input texts be
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regarded as illegal. Likewise, this module's serializer also accepts single
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Python :data:`None`, :class:`bool`, numeric, and :class:`str`
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values as input and will generate output texts consisting solely of a top-level
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JSON null, boolean, number, or string value without raising an exception::
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>>> neither_a_list_nor_a_dict = u"spam and eggs"
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>>> json.dumps(neither_a_list_nor_a_dict) # The result is not a valid JSON text
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'"spam and eggs"'
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This module's serializer does not itself include a way to enforce the
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aforementioned constraint.
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Infinite and NaN Number Values
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The RFC does not permit the representation of infinite or NaN number values.
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Despite that, by default, this module accepts and outputs ``Infinity``,
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``-Infinity``, and ``NaN`` as if they were valid JSON number literal values::
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>>> # Neither of these calls raises an exception, but the results are not valid JSON
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>>> json.dumps(float('-inf'))
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'-Infinity'
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>>> json.dumps(float('nan'))
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'NaN'
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>>> # Same when deserializing
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>>> json.loads('-Infinity')
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-inf
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>>> json.loads('NaN')
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nan
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In the serializer, the *allow_nan* parameter can be used to alter this
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behavior. In the deserializer, the *parse_constant* parameter can be used to
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alter this behavior.
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Repeated Names Within an Object
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The RFC specifies that the names within a JSON object should be unique, but
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does not specify how repeated names in JSON objects should be handled. By
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default, this module does not raise an exception; instead, it ignores all but
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the last name-value pair for a given name::
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>>> weird_json = '{"x": 1, "x": 2, "x": 3}'
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>>> json.loads(weird_json)
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{u'x': 3}
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The *object_pairs_hook* parameter can be used to alter this behavior.
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