564 lines
22 KiB
ReStructuredText
564 lines
22 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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`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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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('\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 io 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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['foo', {'bar': ['baz', None, 1.0, 2]}]
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>>> json.loads('"\\"foo\\bar"')
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'"foo\x08ar'
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>>> from io import StringIO
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>>> io = StringIO('["streaming API"]')
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>>> json.load(io)
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['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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>>> json.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:: bash
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Using json.tool from the shell to validate and pretty-print::
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$ echo '{"json":"obj"}' | python -mjson.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:: python3
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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, default=None, \
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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:`int`, :class:`float`, :class:`bool`,
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``None``) will be skipped instead of raising a :exc:`TypeError`.
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The :mod:`json` module always produces :class:`str` objects, not
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:class:`bytes` objects. Therefore, ``fp.write()`` must support :class:`str`
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input.
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If *ensure_ascii* is ``True`` (the default), the output is guaranteed to
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have all incoming non-ASCII characters escaped. If *ensure_ascii* is
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``False``, these characters will be output as-is.
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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, instead of
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using the JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``).
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If *indent* is a non-negative integer or string, then JSON array elements and
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object members will be pretty-printed with that indent level. An indent level
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of 0, negative, or ``""`` will only insert newlines. ``None`` (the default)
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selects the most compact representation. Using a positive integer indent
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indents that many spaces per level. If *indent* is a string (such as ``"\t"``),
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that string is used to indent each level.
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.. versionchanged:: 3.2
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Allow strings for *indent* in addition to integers.
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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 *separators* is an ``(item_separator, dict_separator)`` tuple, then it
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will be used instead of the default ``(', ', ': ')`` separators. ``(',',
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':')`` is the most compact JSON representation.
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*default(obj)* is a function that should return a serializable version of
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*obj* or raise :exc:`TypeError`. The default simply raises :exc:`TypeError`.
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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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.. 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, default=None, \
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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>`. The arguments have the same meaning as in
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:func:`dump`.
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.. note::
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Unlike :mod:`pickle` and :mod:`marshal`, JSON is not a framed protocol,
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so trying to serialize multiple objects with repeated calls to
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:func:`dump` using the same *fp* will result in an invalid JSON file.
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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, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **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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*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:: 3.1
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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:: 3.1
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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=None, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)
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Deserialize *s* (a :class:`str` instance containing a JSON document) to a
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Python object using this :ref:`conversion table <json-to-py-table>`.
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The other arguments have the same meaning as in :func:`load`, except
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*encoding* which is ignored and deprecated.
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Encoders and Decoders
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---------------------
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.. class:: JSONDecoder(object_hook=None, parse_float=None, parse_int=None, parse_constant=None, strict=True, object_pairs_hook=None)
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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 | str |
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+---------------+-------------------+
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| number (int) | int |
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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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*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:: 3.1
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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` instance
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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` beginning with a
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JSON document) and return a 2-tuple of the Python representation
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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=False, ensure_ascii=True, check_circular=True, allow_nan=True, sort_keys=False, indent=None, separators=None, default=None)
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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 | string |
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+-------------------+---------------+
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| int, 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, 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), the output is guaranteed to
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have all incoming non-ASCII characters escaped. If *ensure_ascii* is
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``False``, these characters will be output as-is.
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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 or string, then JSON array elements and
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object members will be pretty-printed with that indent level. An indent level
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of 0, negative, or ``""`` will only insert newlines. ``None`` (the default)
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selects the most compact representation. Using a positive integer indent
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indents that many spaces per level. If *indent* is a string (such as ``"\t"``),
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that string is used to indent each level.
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.. versionchanged:: 3.2
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Allow strings for *indent* in addition to integers.
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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. The default is ``(', ', ': ')``. To get the most compact JSON
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representation, you should specify ``(',', ':')`` to eliminate whitespace.
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If specified, *default* is a function that gets called for objects that can't
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otherwise be serialized. It should return a JSON encodable version of the
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object or raise a :exc:`TypeError`.
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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):
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try:
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iterable = iter(o)
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except TypeError:
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pass
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else:
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return list(iterable)
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# Let the base class default method raise the TypeError
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return json.JSONEncoder.default(self, o)
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.. method:: encode(o)
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Return a JSON string representation of a Python data structure, *o*. For
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example::
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>>> json.JSONEncoder().encode({"foo": ["bar", "baz"]})
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'{"foo": ["bar", "baz"]}'
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.. method:: iterencode(o)
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Encode the given object, *o*, and yield each string representation as
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available. For example::
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for chunk in json.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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|
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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.
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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
|
|
strings only contain ASCII characters.
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|
|
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Other than the *ensure_ascii* parameter, this module is defined strictly in
|
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terms of conversion between Python objects and
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|
:class:`Unicode strings <str>`, and thus does not otherwise address the issue
|
|
of character encodings.
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|
|
|
|
|
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
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|
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)
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|
'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 = "spam and eggs"
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|
>>> 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)
|
|
{'x': 3}
|
|
|
|
The *object_pairs_hook* parameter can be used to alter this behavior.
|