401 lines
15 KiB
ReStructuredText
401 lines
15 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> is a subset of JavaScript
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syntax (ECMA-262 3rd edition) used as a lightweight data interchange format.
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: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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... 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:: none
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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: line 1 column 2 (char 2)
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.. highlight:: python
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.. note::
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The JSON produced by this module's default settings is a subset of
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YAML, so it may be used as a serializer for that as well.
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Basic Usage
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-----------
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.. function:: dump(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, **kw)
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Serialize *obj* as a JSON formatted stream to *fp* (a ``.write()``-supporting
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file-like object).
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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 *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, then JSON array elements and object
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members will be pretty-printed with that indent level. An indent level of 0,
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or negative, will only insert newlines. ``None`` (the default) selects the
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most compact representation.
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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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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, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, **kw)
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Serialize *obj* to a JSON formatted :class:`str`. The arguments have the
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same meaning as in :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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.. 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 file-like object containing a JSON
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document) to a Python object.
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*object_hook* is an optional function that will be called with the result of
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any object literal decoded (a :class:`dict`). The return value of
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*object_hook* will be used instead of the :class:`dict`. This feature can be used
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to implement custom decoders (e.g. JSON-RPC class hinting).
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*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'``, ``'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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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.
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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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+---------------+-------------------+
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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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.. 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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+-------------------+---------------+
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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 (it is ``None`` by default), then JSON
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array elements and object members will be pretty-printed with that indent
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level. An indent level of 0 will only insert newlines. ``None`` is the most
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compact representation.
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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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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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