624 lines
25 KiB
ReStructuredText
624 lines
25 KiB
ReStructuredText
:mod:`json` --- JSON encoder and decoder
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========================================
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.. module:: json
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:synopsis: Encode and decode the JSON format.
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.. moduleauthor:: Bob Ippolito <bob@redivi.com>
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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>`_, specified by
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:rfc:`7159` (which obsoletes :rfc:`4627`) and by
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`ECMA-404 <http://www.ecma-international.org/publications/standards/Ecma-404.htm>`_,
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is a lightweight data interchange format inspired by
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`JavaScript <https://en.wikipedia.org/wiki/JavaScript>`_ object literal syntax
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(although it is not a strict subset of JavaScript [#rfc-errata]_ ).
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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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Encoding basic Python object hierarchies::
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>>> import json
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>>> json.dumps(['foo', {'bar': ('baz', None, 1.0, 2)}])
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'["foo", {"bar": ["baz", null, 1.0, 2]}]'
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>>> print json.dumps("\"foo\bar")
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"\"foo\bar"
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>>> print json.dumps(u'\u1234')
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"\u1234"
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>>> print json.dumps('\\')
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"\\"
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>>> print json.dumps({"c": 0, "b": 0, "a": 0}, sort_keys=True)
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{"a": 0, "b": 0, "c": 0}
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>>> from StringIO import StringIO
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>>> io = StringIO()
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>>> json.dump(['streaming API'], io)
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>>> io.getvalue()
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'["streaming API"]'
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Compact encoding::
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>>> import json
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>>> json.dumps([1,2,3,{'4': 5, '6': 7}], separators=(',',':'))
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'[1,2,3,{"4":5,"6":7}]'
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Pretty printing::
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>>> import json
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>>> print json.dumps({'4': 5, '6': 7}, sort_keys=True,
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... indent=4, separators=(',', ': '))
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{
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"4": 5,
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"6": 7
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}
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Decoding JSON::
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>>> import json
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>>> json.loads('["foo", {"bar":["baz", null, 1.0, 2]}]')
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[u'foo', {u'bar': [u'baz', None, 1.0, 2]}]
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>>> json.loads('"\\"foo\\bar"')
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u'"foo\x08ar'
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>>> from StringIO import StringIO
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>>> io = StringIO('["streaming API"]')
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>>> json.load(io)
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[u'streaming API']
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Specializing JSON object decoding::
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>>> import json
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>>> def as_complex(dct):
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... if '__complex__' in dct:
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... return complex(dct['real'], dct['imag'])
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... return dct
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...
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>>> json.loads('{"__complex__": true, "real": 1, "imag": 2}',
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... object_hook=as_complex)
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(1+2j)
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>>> import decimal
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>>> json.loads('1.1', parse_float=decimal.Decimal)
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Decimal('1.1')
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Extending :class:`JSONEncoder`::
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>>> import json
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>>> class ComplexEncoder(json.JSONEncoder):
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... def default(self, obj):
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... if isinstance(obj, complex):
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... return [obj.real, obj.imag]
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... # Let the base class default method raise the TypeError
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... return json.JSONEncoder.default(self, obj)
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...
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>>> dumps(2 + 1j, cls=ComplexEncoder)
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'[2.0, 1.0]'
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>>> ComplexEncoder().encode(2 + 1j)
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'[2.0, 1.0]'
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>>> list(ComplexEncoder().iterencode(2 + 1j))
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['[', '2.0', ', ', '1.0', ']']
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.. highlight:: none
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Using :mod:`json.tool` from the shell to validate and pretty-print::
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$ echo '{"json":"obj"}' | python -m json.tool
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{
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"json": "obj"
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}
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$ echo '{1.2:3.4}' | python -mjson.tool
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Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
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.. highlight:: python
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.. note::
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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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-----------
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.. function:: dump(obj, fp, skipkeys=False, ensure_ascii=True, \
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check_circular=True, allow_nan=True, cls=None, \
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indent=None, separators=None, encoding="utf-8", \
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default=None, sort_keys=False, **kw)
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Serialize *obj* as a JSON formatted stream to *fp* (a ``.write()``-supporting
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:term:`file-like object`) using this :ref:`conversion table
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<py-to-json-table>`.
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If *skipkeys* is true (default: ``False``), then dict keys that are not
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of a basic type (:class:`str`, :class:`unicode`, :class:`int`, :class:`long`,
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:class:`float`, :class:`bool`, ``None``) will be skipped instead of raising a
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:exc:`TypeError`.
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If *ensure_ascii* is true (the default), all non-ASCII characters in the
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output are escaped with ``\uXXXX`` sequences, and the result is a
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:class:`str` instance consisting of ASCII characters only. If
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*ensure_ascii* is false, some chunks written to *fp* may be
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:class:`unicode` instances. This usually happens because the input contains
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unicode strings or the *encoding* parameter is used. Unless ``fp.write()``
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explicitly understands :class:`unicode` (as in :func:`codecs.getwriter`)
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this is likely to cause an error.
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If *check_circular* is false (default: ``True``), then the circular
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reference check for container types will be skipped and a circular reference
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will result in an :exc:`OverflowError` (or worse).
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If *allow_nan* is false (default: ``True``), then it will be a
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:exc:`ValueError` to serialize out of range :class:`float` values (``nan``,
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``inf``, ``-inf``) in strict compliance of the JSON specification.
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If *allow_nan* is true, their JavaScript equivalents (``NaN``,
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``Infinity``, ``-Infinity``) will be used.
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If *indent* is a non-negative integer, then JSON array elements and object
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members will be pretty-printed with that indent level. An indent level of 0,
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or negative, will only insert newlines. ``None`` (the default) selects the
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most compact representation.
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.. note::
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Since the default item separator is ``', '``, the output might include
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trailing whitespace when *indent* is specified. You can use
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``separators=(',', ': ')`` to avoid this.
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If specified, *separators* should be an ``(item_separator, key_separator)``
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tuple. By default, ``(', ', ': ')`` are used. To get the most compact JSON
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representation, you should specify ``(',', ':')`` to eliminate whitespace.
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*encoding* is the character encoding for str instances, default is UTF-8.
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If specified, *default* should be a function that gets called for objects that
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can't otherwise be serialized. It should return a JSON encodable version of
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the object or raise a :exc:`TypeError`. If not specified, :exc:`TypeError`
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is raised.
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If *sort_keys* is true (default: ``False``), then the output of
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dictionaries will be sorted by key.
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To use a custom :class:`JSONEncoder` subclass (e.g. one that overrides the
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:meth:`default` method to serialize additional types), specify it with the
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*cls* kwarg; otherwise :class:`JSONEncoder` is used.
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.. note::
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Unlike :mod:`pickle` and :mod:`marshal`, JSON is not a framed protocol so
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trying to serialize more objects with repeated calls to :func:`dump` and
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the same *fp* will result in an invalid JSON file.
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.. function:: dumps(obj, skipkeys=False, ensure_ascii=True, \
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check_circular=True, allow_nan=True, cls=None, \
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indent=None, separators=None, encoding="utf-8", \
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default=None, sort_keys=False, **kw)
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Serialize *obj* to a JSON formatted :class:`str` using this :ref:`conversion
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table <py-to-json-table>`. If *ensure_ascii* is false, the result may
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contain non-ASCII characters and the return value may be a :class:`unicode`
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instance.
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The arguments have the same meaning as in :func:`dump`.
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.. note::
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Keys in key/value pairs of JSON are always of the type :class:`str`. When
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a dictionary is converted into JSON, all the keys of the dictionary are
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coerced to strings. As a result of this, if a dictionary is converted
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into JSON and then back into a dictionary, the dictionary may not equal
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the original one. That is, ``loads(dumps(x)) != x`` if x has non-string
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keys.
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.. function:: load(fp[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[, **kw]]]]]]]])
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Deserialize *fp* (a ``.read()``-supporting :term:`file-like object`
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containing a JSON document) to a Python object using this :ref:`conversion
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table <json-to-py-table>`.
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If the contents of *fp* are encoded with an ASCII based encoding other than
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UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be specified.
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Encodings that are not ASCII based (such as UCS-2) are not allowed, and
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should be wrapped with ``codecs.getreader(encoding)(fp)``, or simply decoded
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to a :class:`unicode` object and passed to :func:`loads`.
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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 <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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return value of *object_pairs_hook* will be used instead of the
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:class:`dict`. This feature can be used to implement custom decoders that
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rely on the order that the key and value pairs are decoded (for example,
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:func:`collections.OrderedDict` will remember the order of insertion). If
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*object_hook* is also defined, the *object_pairs_hook* takes priority.
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.. versionchanged:: 2.7
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Added support for *object_pairs_hook*.
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*parse_float*, if specified, will be called with the string of every JSON
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float to be decoded. By default, this is equivalent to ``float(num_str)``.
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This can be used to use another datatype or parser for JSON floats
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(e.g. :class:`decimal.Decimal`).
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*parse_int*, if specified, will be called with the string of every JSON int
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to be decoded. By default, this is equivalent to ``int(num_str)``. This can
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be used to use another datatype or parser for JSON integers
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(e.g. :class:`float`).
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*parse_constant*, if specified, will be called with one of the following
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strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.
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This can be used to raise an exception if invalid JSON numbers
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are encountered.
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.. versionchanged:: 2.7
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*parse_constant* doesn't get called on 'null', 'true', 'false' anymore.
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To use a custom :class:`JSONDecoder` subclass, specify it with the ``cls``
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kwarg; otherwise :class:`JSONDecoder` is used. Additional keyword arguments
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will be passed to the constructor of the class.
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.. function:: loads(s[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[, **kw]]]]]]]])
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Deserialize *s* (a :class:`str` or :class:`unicode` instance containing a JSON
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document) to a Python object using this :ref:`conversion table
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<json-to-py-table>`.
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If *s* is a :class:`str` instance and is encoded with an ASCII based encoding
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other than UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be
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specified. Encodings that are not ASCII based (such as UCS-2) are not
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allowed and should be decoded to :class:`unicode` first.
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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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---------------------
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.. class:: JSONDecoder([encoding[, object_hook[, parse_float[, parse_int[, parse_constant[, strict[, object_pairs_hook]]]]]]])
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Simple JSON decoder.
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Performs the following translations in decoding by default:
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.. _json-to-py-table:
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+---------------+-------------------+
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| JSON | Python |
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+===============+===================+
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| object | dict |
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+---------------+-------------------+
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| array | list |
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+---------------+-------------------+
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| string | unicode |
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+---------------+-------------------+
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| number (int) | int, long |
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+---------------+-------------------+
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| number (real) | float |
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+---------------+-------------------+
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| true | True |
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+---------------+-------------------+
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| false | False |
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+---------------+-------------------+
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| null | None |
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+---------------+-------------------+
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It also understands ``NaN``, ``Infinity``, and ``-Infinity`` as their
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corresponding ``float`` values, which is outside the JSON spec.
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*encoding* determines the encoding used to interpret any :class:`str` objects
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decoded by this instance (UTF-8 by default). It has no effect when decoding
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:class:`unicode` objects.
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Note that currently only encodings that are a superset of ASCII work, strings
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of other encodings should be passed in as :class:`unicode`.
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*object_hook*, if specified, will be called with the result of every JSON
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object decoded and its return value will be used in place of the given
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:class:`dict`. This can be used to provide custom deserializations (e.g. to
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support JSON-RPC class hinting).
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*object_pairs_hook*, if specified will be called with the result of every
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JSON object decoded with an ordered list of pairs. The return value of
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*object_pairs_hook* will be used instead of the :class:`dict`. This
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feature can be used to implement custom decoders that rely on the order
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that the key and value pairs are decoded (for example,
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:func:`collections.OrderedDict` will remember the order of insertion). If
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*object_hook* is also defined, the *object_pairs_hook* takes priority.
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.. versionchanged:: 2.7
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Added support for *object_pairs_hook*.
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*parse_float*, if specified, will be called with the string of every JSON
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float to be decoded. By default, this is equivalent to ``float(num_str)``.
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This can be used to use another datatype or parser for JSON floats
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(e.g. :class:`decimal.Decimal`).
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*parse_int*, if specified, will be called with the string of every JSON int
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to be decoded. By default, this is equivalent to ``int(num_str)``. This can
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be used to use another datatype or parser for JSON integers
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(e.g. :class:`float`).
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*parse_constant*, if specified, will be called with one of the following
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strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``, ``'null'``, ``'true'``,
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``'false'``. This can be used to raise an exception if invalid JSON numbers
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are encountered.
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If *strict* is false (``True`` is the default), then control characters
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will be allowed inside strings. Control characters in this context are
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those with character codes in the 0-31 range, including ``'\t'`` (tab),
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``'\n'``, ``'\r'`` and ``'\0'``.
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If the data being deserialized is not a valid JSON document, a
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:exc:`ValueError` will be raised.
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.. method:: decode(s)
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Return the Python representation of *s* (a :class:`str` or
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:class:`unicode` instance containing a JSON document).
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.. method:: raw_decode(s)
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Decode a JSON document from *s* (a :class:`str` or :class:`unicode`
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beginning with a JSON document) and return a 2-tuple of the Python
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representation and the index in *s* where the document ended.
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This can be used to decode a JSON document from a string that may have
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extraneous data at the end.
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.. class:: JSONEncoder([skipkeys[, ensure_ascii[, check_circular[, allow_nan[, sort_keys[, indent[, separators[, encoding[, default]]]]]]]]])
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Extensible JSON encoder for Python data structures.
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Supports the following objects and types by default:
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.. _py-to-json-table:
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+-------------------+---------------+
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| Python | JSON |
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+===================+===============+
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| dict | object |
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+-------------------+---------------+
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| list, tuple | array |
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+-------------------+---------------+
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| str, unicode | string |
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+-------------------+---------------+
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| int, long, float | number |
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+-------------------+---------------+
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| True | true |
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+-------------------+---------------+
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| False | false |
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+-------------------+---------------+
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| None | null |
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+-------------------+---------------+
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To extend this to recognize other objects, subclass and implement a
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:meth:`default` method with another method that returns a serializable object
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for ``o`` if possible, otherwise it should call the superclass implementation
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(to raise :exc:`TypeError`).
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If *skipkeys* is false (the default), then it is a :exc:`TypeError` to
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attempt encoding of keys that are not str, int, long, float or ``None``. If
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*skipkeys* is true, such items are simply skipped.
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If *ensure_ascii* is true (the default), all non-ASCII characters in the
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output are escaped with ``\uXXXX`` sequences, and the results are
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:class:`str` instances consisting of ASCII characters only. If
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|
*ensure_ascii* is false, a result may be a :class:`unicode`
|
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instance. This usually happens if the input contains unicode strings or the
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*encoding* parameter is used.
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If *check_circular* is true (the default), then lists, dicts, and custom
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encoded objects will be checked for circular references during encoding to
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prevent an infinite recursion (which would cause an :exc:`OverflowError`).
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Otherwise, no such check takes place.
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If *allow_nan* is true (the default), then ``NaN``, ``Infinity``, and
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``-Infinity`` will be encoded as such. This behavior is not JSON
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specification compliant, but is consistent with most JavaScript based
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encoders and decoders. Otherwise, it will be a :exc:`ValueError` to encode
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such floats.
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If *sort_keys* is true (default: ``False``), then the output of dictionaries
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will be sorted by key; this is useful for regression tests to ensure that
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JSON serializations can be compared on a day-to-day basis.
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If *indent* is a non-negative integer (it is ``None`` by default), then JSON
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array elements and object members will be pretty-printed with that indent
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level. An indent level of 0 will only insert newlines. ``None`` is the most
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compact representation.
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.. note::
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Since the default item separator is ``', '``, the output might include
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trailing whitespace when *indent* is specified. You can use
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``separators=(',', ': ')`` to avoid this.
|
|
|
|
If specified, *separators* should be an ``(item_separator, key_separator)``
|
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tuple. By default, ``(', ', ': ')`` are used. To get the most compact JSON
|
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representation, you should specify ``(',', ':')`` to eliminate whitespace.
|
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If specified, *default* should be a function that gets called for objects that
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can't otherwise be serialized. It should return a JSON encodable version of
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the object or raise a :exc:`TypeError`. If not specified, :exc:`TypeError`
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is raised.
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If *encoding* is not ``None``, then all input strings will be transformed
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into unicode using that encoding prior to JSON-encoding. The default is
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UTF-8.
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.. method:: default(o)
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Implement this method in a subclass such that it returns a serializable
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object for *o*, or calls the base implementation (to raise a
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:exc:`TypeError`).
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For example, to support arbitrary iterators, you could implement default
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like this::
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|
|
|
def default(self, o):
|
|
try:
|
|
iterable = iter(o)
|
|
except TypeError:
|
|
pass
|
|
else:
|
|
return list(iterable)
|
|
# Let the base class default method raise the TypeError
|
|
return JSONEncoder.default(self, o)
|
|
|
|
|
|
.. method:: encode(o)
|
|
|
|
Return a JSON string representation of a Python data structure, *o*. For
|
|
example::
|
|
|
|
>>> JSONEncoder().encode({"foo": ["bar", "baz"]})
|
|
'{"foo": ["bar", "baz"]}'
|
|
|
|
|
|
.. method:: iterencode(o)
|
|
|
|
Encode the given object, *o*, and yield each string representation as
|
|
available. For example::
|
|
|
|
for chunk in JSONEncoder().iterencode(bigobject):
|
|
mysocket.write(chunk)
|
|
|
|
|
|
Standard Compliance and Interoperability
|
|
----------------------------------------
|
|
|
|
The JSON format is specified by :rfc:`7159` and by
|
|
`ECMA-404 <http://www.ecma-international.org/publications/standards/Ecma-404.htm>`_.
|
|
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:
|
|
|
|
- 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 requires that JSON be represented using either UTF-8, UTF-16, or
|
|
UTF-32, with UTF-8 being the recommended default for maximum interoperability.
|
|
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.
|
|
|
|
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.
|
|
|
|
The RFC prohibits adding a byte order mark (BOM) to the start of a JSON text,
|
|
and this module's serializer does not add a BOM to its output.
|
|
The RFC permits, but does not require, JSON deserializers to ignore an initial
|
|
BOM in their input. This module's deserializer raises a :exc:`ValueError`
|
|
when an initial BOM is present.
|
|
|
|
The RFC does not explicitly forbid JSON strings which contain byte sequences
|
|
that don't correspond to valid Unicode characters (e.g. unpaired UTF-16
|
|
surrogates), but it does note that they may cause interoperability problems.
|
|
By default, this module accepts and outputs (when present in the original
|
|
:class:`str`) code points for such sequences.
|
|
|
|
|
|
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 mandate 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.
|
|
|
|
|
|
Top-level Non-Object, Non-Array Values
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The old version of JSON specified by the obsolete :rfc:`4627` required that
|
|
the top-level value of a JSON text must be either a JSON object or array
|
|
(Python :class:`dict` or :class:`list`), and could not be a JSON null,
|
|
boolean, number, or string value. :rfc:`7159` removed that restriction, and
|
|
this module does not and has never implemented that restriction in either its
|
|
serializer or its deserializer.
|
|
|
|
Regardless, for maximum interoperability, you may wish to voluntarily adhere
|
|
to the restriction yourself.
|
|
|
|
|
|
Implementation Limitations
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Some JSON deserializer implementations may set limits on:
|
|
|
|
* the size of accepted JSON texts
|
|
* the maximum level of nesting of JSON objects and arrays
|
|
* the range and precision of JSON numbers
|
|
* the content and maximum length of JSON strings
|
|
|
|
This module does not impose any such limits beyond those of the relevant
|
|
Python datatypes themselves or the Python interpreter itself.
|
|
|
|
When serializing to JSON, beware any such limitations in applications that may
|
|
consume your JSON. In particular, it is common for JSON numbers to be
|
|
deserialized into IEEE 754 double precision numbers and thus subject to that
|
|
representation's range and precision limitations. This is especially relevant
|
|
when serializing Python :class:`int` values of extremely large magnitude, or
|
|
when serializing instances of "exotic" numerical types such as
|
|
:class:`decimal.Decimal`.
|
|
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#rfc-errata] As noted in `the errata for RFC 7159
|
|
<https://www.rfc-editor.org/errata_search.php?rfc=7159>`_,
|
|
JSON permits literal U+2028 (LINE SEPARATOR) and
|
|
U+2029 (PARAGRAPH SEPARATOR) characters in strings, whereas JavaScript
|
|
(as of ECMAScript Edition 5.1) does not.
|