696 lines
26 KiB
ReStructuredText
696 lines
26 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:`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('\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, indent=4))
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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 -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 -m json.tool
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Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
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See :ref:`json-commandline` for detailed documentation.
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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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If specified, *separators* should be an ``(item_separator, key_separator)``
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tuple. The default is ``(', ', ': ')`` if *indent* is ``None`` and
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``(',', ': ')`` otherwise. To get the most compact JSON representation,
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you should specify ``(',', ':')`` to eliminate whitespace.
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.. versionchanged:: 3.4
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Use ``(',', ': ')`` as default if *indent* is not ``None``.
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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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If the data being deserialized is not a valid JSON document, a
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:exc:`JSONDecodeError` will be raised.
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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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If the data being deserialized is not a valid JSON document, a
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:exc:`JSONDecodeError` will be raised.
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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:`JSONDecodeError` 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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:exc:`JSONDecodeError` will be raised if the given JSON document is not
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valid.
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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, int- & float-derived Enums | 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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.. versionchanged:: 3.4
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Added support for int- and float-derived Enum classes.
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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)
|
|
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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|
|
If specified, *separators* should be an ``(item_separator, key_separator)``
|
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tuple. The default is ``(', ', ': ')`` if *indent* is ``None`` and
|
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``(',', ': ')`` otherwise. To get the most compact JSON representation,
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you should specify ``(',', ':')`` to eliminate whitespace.
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.. versionchanged:: 3.4
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Use ``(',', ': ')`` as default if *indent* is not ``None``.
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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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Exceptions
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----------
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.. exception:: JSONDecodeError(msg, doc, pos, end=None)
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Subclass of :exc:`ValueError` with the following additional attributes:
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.. attribute:: msg
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The unformatted error message.
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.. attribute:: doc
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The JSON document being parsed.
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.. attribute:: pos
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The start index of *doc* where parsing failed.
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.. attribute:: lineno
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The line corresponding to *pos*.
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.. attribute:: colno
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The column corresponding to *pos*.
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.. versionadded:: 3.5
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Standard Compliance and Interoperability
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----------------------------------------
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The JSON format is specified by :rfc:`7159` and by
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`ECMA-404 <http://www.ecma-international.org/publications/standards/Ecma-404.htm>`_.
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This section details this module's level of compliance with the RFC.
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For simplicity, :class:`JSONEncoder` and :class:`JSONDecoder` subclasses, and
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parameters other 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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- 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 requires that JSON be represented using either UTF-8, UTF-16, or
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UTF-32, with UTF-8 being the recommended default for maximum interoperability.
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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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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 directly address
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the issue of character encodings.
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The RFC prohibits adding a byte order mark (BOM) to the start of a JSON text,
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and this module's serializer does not add a BOM to its output.
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The RFC permits, but does not require, JSON deserializers to ignore an initial
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BOM in their input. This module's deserializer raises a :exc:`ValueError`
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when an initial BOM is present.
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The RFC does not explicitly forbid JSON strings which contain byte sequences
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that don't correspond to valid Unicode characters (e.g. unpaired UTF-16
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surrogates), but it does note that they may cause interoperability problems.
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By default, this module accepts and outputs (when present in the original
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:class:`str`) code points for such sequences.
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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 mandate 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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{'x': 3}
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The *object_pairs_hook* parameter can be used to alter this behavior.
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Top-level Non-Object, Non-Array Values
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The old version of JSON specified by the obsolete :rfc:`4627` required that
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the top-level value of a JSON text must be either a JSON object or array
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(Python :class:`dict` or :class:`list`), and could not be a JSON null,
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boolean, number, or string value. :rfc:`7159` removed that restriction, and
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this module does not and has never implemented that restriction in either its
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serializer or its deserializer.
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Regardless, for maximum interoperability, you may wish to voluntarily adhere
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to the restriction yourself.
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Implementation Limitations
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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Some JSON deserializer implementations may set limits on:
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* the size of accepted JSON texts
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* the maximum level of nesting of JSON objects and arrays
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* the range and precision of JSON numbers
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* the content and maximum length of JSON strings
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This module does not impose any such limits beyond those of the relevant
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Python datatypes themselves or the Python interpreter itself.
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When serializing to JSON, beware any such limitations in applications that may
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consume your JSON. In particular, it is common for JSON numbers to be
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deserialized into IEEE 754 double precision numbers and thus subject to that
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representation's range and precision limitations. This is especially relevant
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when serializing Python :class:`int` values of extremely large magnitude, or
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when serializing instances of "exotic" numerical types such as
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:class:`decimal.Decimal`.
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.. highlight:: bash
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.. module:: json.tool
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.. _json-commandline:
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Command Line Interface
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----------------------
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The :mod:`json.tool` module provides a simple command line interface to validate
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and pretty-print JSON objects.
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If the optional ``infile`` and ``outfile`` arguments are not
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specified, :attr:`sys.stdin` and :attr:`sys.stdout` will be used respectively::
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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 -m json.tool
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Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
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.. versionchanged:: 3.5
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The output is now in the same order as the input. Use the
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:option:`--sort-keys` option to sort the output of dictionaries
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alphabetically by key.
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Command line options
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^^^^^^^^^^^^^^^^^^^^
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.. cmdoption:: infile
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The JSON file to be validated or pretty-printed::
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$ python -m json.tool mp_films.json
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[
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{
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"title": "And Now for Something Completely Different",
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"year": 1971
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},
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{
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"title": "Monty Python and the Holy Grail",
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"year": 1975
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}
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]
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If *infile* is not specified, read from :attr:`sys.stdin`.
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.. cmdoption:: outfile
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Write the output of the *infile* to the given *outfile*. Otherwise, write it
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to :attr:`sys.stdout`.
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.. cmdoption:: --sort-keys
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Sort the output of dictionaries alphabetically by key.
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.. versionadded:: 3.5
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.. cmdoption:: -h, --help
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Show the help message.
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.. rubric:: Footnotes
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.. [#rfc-errata] As noted in `the errata for RFC 7159
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<http://www.rfc-editor.org/errata_search.php?rfc=7159>`_,
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JSON permits literal U+2028 (LINE SEPARATOR) and
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U+2029 (PARAGRAPH SEPARATOR) characters in strings, whereas JavaScript
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(as of ECMAScript Edition 5.1) does not.
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