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:mod:`simplejson` --- JSON encoder and decoder

.. module:: simplejson
   :synopsis: Encode and decode the JSON format.
.. moduleauthor:: Bob Ippolito <bob@redivi.com>
.. sectionauthor:: Bob Ippolito <bob@redivi.com>

JSON (JavaScript Object Notation), specified by RFC 7159 (which obsoletes RFC 4627) and by ECMA-404, is a lightweight data interchange format inspired by JavaScript object literal syntax (although it is not a strict subset of JavaScript [1] ).

:mod:`simplejson` exposes an API familiar to users of the standard library :mod:`marshal` and :mod:`pickle` modules. It is the externally maintained version of the :mod:`json` library contained in Python 2.6, but maintains compatibility with Python 2.5 and (currently) has significant performance advantages, even without using the optional C extension for speedups. :mod:`simplejson` is also supported on Python 3.3+.

Development of simplejson happens on Github: http://github.com/simplejson/simplejson

Encoding basic Python object hierarchies:

>>> import simplejson as json
>>> json.dumps(['foo', {'bar': ('baz', None, 1.0, 2)}])
'["foo", {"bar": ["baz", null, 1.0, 2]}]'
>>> print(json.dumps("\"foo\bar"))
"\"foo\bar"
>>> print(json.dumps(u'\u1234'))
"\u1234"
>>> print(json.dumps('\\'))
"\\"
>>> print(json.dumps({"c": 0, "b": 0, "a": 0}, sort_keys=True))
{"a": 0, "b": 0, "c": 0}
>>> from simplejson.compat import StringIO
>>> io = StringIO()
>>> json.dump(['streaming API'], io)
>>> io.getvalue()
'["streaming API"]'

Compact encoding:

>>> import simplejson as json
>>> obj = [1,2,3,{'4': 5, '6': 7}]
>>> json.dumps(obj, separators=(',', ':'), sort_keys=True)
'[1,2,3,{"4":5,"6":7}]'

Pretty printing:

>>> import simplejson as json
>>> print(json.dumps({'4': 5, '6': 7}, sort_keys=True, indent=4 * ' '))
{
    "4": 5,
    "6": 7
}

Decoding JSON:

>>> import simplejson as json
>>> obj = [u'foo', {u'bar': [u'baz', None, 1.0, 2]}]
>>> json.loads('["foo", {"bar":["baz", null, 1.0, 2]}]') == obj
True
>>> json.loads('"\\"foo\\bar"') == u'"foo\x08ar'
True
>>> from simplejson.compat import StringIO
>>> io = StringIO('["streaming API"]')
>>> json.load(io)[0] == 'streaming API'
True

Using Decimal instead of float:

>>> import simplejson as json
>>> from decimal import Decimal
>>> json.loads('1.1', use_decimal=True) == Decimal('1.1')
True
>>> json.dumps(Decimal('1.1'), use_decimal=True) == '1.1'
True

Specializing JSON object decoding:

>>> import simplejson as json
>>> def as_complex(dct):
...     if '__complex__' in dct:
...         return complex(dct['real'], dct['imag'])
...     return dct
...
>>> json.loads('{"__complex__": true, "real": 1, "imag": 2}',
...     object_hook=as_complex)
(1+2j)
>>> import decimal
>>> json.loads('1.1', parse_float=decimal.Decimal) == decimal.Decimal('1.1')
True

Specializing JSON object encoding:

>>> import simplejson as json
>>> def encode_complex(obj):
...     if isinstance(obj, complex):
...         return [obj.real, obj.imag]
...     raise TypeError(repr(obj) + " is not JSON serializable")
...
>>> json.dumps(2 + 1j, default=encode_complex)
'[2.0, 1.0]'
>>> json.JSONEncoder(default=encode_complex).encode(2 + 1j)
'[2.0, 1.0]'
>>> ''.join(json.JSONEncoder(default=encode_complex).iterencode(2 + 1j))
'[2.0, 1.0]'

Using :mod:`simplejson.tool` from the shell to validate and pretty-print:

$ echo '{"json":"obj"}' | python -m simplejson.tool
{
    "json": "obj"
}
$ echo '{ 1.2:3.4}' | python -m simplejson.tool
Expecting property name enclosed in double quotes: line 1 column 3 (char 2)

Note

JSON is a subset of YAML 1.2. The JSON produced by this module's default settings (in particular, the default separators value) is also a subset of YAML 1.0 and 1.1. This module can thus also be used as a YAML serializer.

Basic Usage

.. function:: dump(obj, fp, skipkeys=False, ensure_ascii=True, \
                   check_circular=True, allow_nan=True, cls=None, \
                   indent=None, separators=None, encoding='utf-8', \
                   default=None, use_decimal=True, \
                   namedtuple_as_object=True, tuple_as_array=True, \
                   bigint_as_string=False, sort_keys=False, \
                   item_sort_key=None, for_json=None, ignore_nan=False, \
                   int_as_string_bitcount=None, iterable_as_array=False, **kw)

   Serialize *obj* as a JSON formatted stream to *fp* (a ``.write()``-supporting
   file-like object) using this :ref:`conversion table <py-to-json-table>`.

   If *skipkeys* is true (default: ``False``), then dict keys that are not
   of a basic type (:class:`str`, :class:`unicode`, :class:`int`, :class:`long`,
   :class:`float`, :class:`bool`, ``None``) will be skipped instead of raising a
   :exc:`TypeError`.

   The :mod:`simplejson` module will produce :class:`str` objects in Python 3,
   not :class:`bytes` objects. Therefore, ``fp.write()`` must support
   :class:`str` input.

   If *ensure_ascii* is false (default: ``True``), then some chunks written
   to *fp* may be :class:`unicode` instances, subject to normal Python
   :class:`str` to :class:`unicode` coercion rules.  Unless ``fp.write()``
   explicitly understands :class:`unicode` (as in :func:`codecs.getwriter`) this
   is likely to cause an error. It's best to leave the default settings, because
   they are safe and it is highly optimized.

   If *check_circular* is false (default: ``True``), then the circular
   reference check for container types will be skipped and a circular reference
   will result in an :exc:`OverflowError` (or worse).

   If *allow_nan* is false (default: ``True``), then it will be a
   :exc:`ValueError` to serialize out of range :class:`float` values (``nan``,
   ``inf``, ``-inf``) in strict compliance of the original JSON specification.
   If *allow_nan* is true, their JavaScript equivalents will be used
   (``NaN``, ``Infinity``, ``-Infinity``). See also *ignore_nan* for ECMA-262
   compliant behavior.

   If *indent* is a string, then JSON array elements and object members
   will be pretty-printed with a newline followed by that string repeated
   for each level of nesting. ``None`` (the default) selects the most compact
   representation without any newlines. For backwards compatibility with
   versions of simplejson earlier than 2.1.0, an integer is also accepted
   and is converted to a string with that many spaces.

   .. versionchanged:: 2.1.0
      Changed *indent* from an integer number of spaces to a string.

   If specified, *separators* should be an ``(item_separator, key_separator)``
   tuple.  The default is ``(', ', ': ')`` if *indent* is ``None`` and
   ``(',', ': ')`` otherwise.  To get the most compact JSON representation,
   you should specify ``(',', ':')`` to eliminate whitespace.

   .. versionchanged:: 2.1.4
      Use ``(',', ': ')`` as default if *indent* is not ``None``.

   If *encoding* is not ``None``, then all input :class:`bytes` objects in
   Python 3 and 8-bit strings in Python 2 will be transformed
   into unicode using that encoding prior to JSON-encoding.  The default is
   ``'utf-8'``.  If *encoding* is ``None``, then all :class:`bytes` objects
   will be passed to the *default* function in Python 3

   .. versionchanged:: 3.15.0
      ``encoding=None`` disables serializing :class:`bytes` by default in
      Python 3.


   *default(obj)* is a function that should return a serializable version of
   *obj* or raise :exc:`TypeError`. The default simply raises :exc:`TypeError`.

   To use a custom :class:`JSONEncoder` subclass (e.g. one that overrides the
   :meth:`default` method to serialize additional types), specify it with the
   *cls* kwarg.

   .. note::

        Subclassing is not recommended. Use the *default* kwarg
        or *for_json* instead. This is faster and more portable.

   If *use_decimal* is true (default: ``True``) then :class:`decimal.Decimal`
   will be natively serialized to JSON with full precision.

   .. versionchanged:: 2.1.0
      *use_decimal* is new in 2.1.0.

   .. versionchanged:: 2.2.0
      The default of *use_decimal* changed to ``True`` in 2.2.0.

   If *namedtuple_as_object* is true (default: ``True``),
   objects with ``_asdict()`` methods will be encoded
   as JSON objects.

   .. versionchanged:: 2.2.0
     *namedtuple_as_object* is new in 2.2.0.

   .. versionchanged:: 2.3.0
     *namedtuple_as_object* no longer requires that these objects be
     subclasses of :class:`tuple`.

   If *tuple_as_array* is true (default: ``True``),
   :class:`tuple` (and subclasses) will be encoded as JSON arrays.

  If *iterable_as_array* is true (default: ``False``),
  any object not in the above table that implements ``__iter__()``
  will be encoded as a JSON array.

  .. versionchanged:: 3.8.0
    *iterable_as_array* is new in 3.8.0.

   .. versionchanged:: 2.2.0
     *tuple_as_array* is new in 2.2.0.

   If *bigint_as_string* is true (default: ``False``), :class:`int` ``2**53``
   and higher or lower than ``-2**53`` will be encoded as strings. This is to
   avoid the rounding that happens in Javascript otherwise. Note that this
   option loses type information, so use with extreme caution.
   See also *int_as_string_bitcount*.

   .. versionchanged:: 2.4.0
     *bigint_as_string* is new in 2.4.0.

   If *sort_keys* is true (not the default), then the output of dictionaries
   will be sorted by key; this is useful for regression tests to ensure that
   JSON serializations can be compared on a day-to-day basis.

   .. versionchanged:: 3.0.0
      Sorting now happens after the keys have been coerced to
      strings, to avoid comparison of heterogeneously typed objects
      (since this does not work in Python 3.3+)

   If *item_sort_key* is a callable (not the default), then the output of
   dictionaries will be sorted with it. The callable will be used like this:
   ``sorted(dct.items(), key=item_sort_key)``. This option takes precedence
   over *sort_keys*.

   .. versionchanged:: 2.5.0
      *item_sort_key* is new in 2.5.0.

   .. versionchanged:: 3.0.0
      Sorting now happens after the keys have been coerced to
      strings, to avoid comparison of heterogeneously typed objects
      (since this does not work in Python 3.3+)

   If *for_json* is true (not the default), objects with a ``for_json()``
   method will use the return value of that method for encoding as JSON instead
   of the object.

   .. versionchanged:: 3.2.0
      *for_json* is new in 3.2.0.

   If *ignore_nan* is true (default: ``False``), then out of range
   :class:`float` values (``nan``, ``inf``, ``-inf``) will be serialized as
   ``null`` in compliance with the ECMA-262 specification. If true, this will
   override *allow_nan*.

   .. versionchanged:: 3.2.0
      *ignore_nan* is new in 3.2.0.

   If *int_as_string_bitcount* is a positive number ``n`` (default: ``None``),
   :class:`int` ``2**n`` and higher or lower than ``-2**n`` will be encoded as strings. This is to
   avoid the rounding that happens in Javascript otherwise. Note that this
   option loses type information, so use with extreme caution.
   See also *bigint_as_string* (which is equivalent to `int_as_string_bitcount=53`).

   .. versionchanged:: 3.5.0
     *int_as_string_bitcount* is new in 3.5.0.

   .. note::

        JSON is not a framed protocol so unlike :mod:`pickle` or :mod:`marshal` it
        does not make sense to serialize more than one JSON document without some
        container protocol to delimit them.

.. function:: dumps(obj, skipkeys=False, ensure_ascii=True, \
                    check_circular=True, allow_nan=True, cls=None, \
                    indent=None, separators=None, encoding='utf-8', \
                    default=None, use_decimal=True, \
                    namedtuple_as_object=True, tuple_as_array=True, \
                    bigint_as_string=False, sort_keys=False, \
                    item_sort_key=None, for_json=None, ignore_nan=False, \
                    int_as_string_bitcount=None, iterable_as_array=False, **kw)

   Serialize *obj* to a JSON formatted :class:`str`.

   If *ensure_ascii* is false, then the return value will be a
   :class:`unicode` instance.  The other arguments have the same meaning as in
   :func:`dump`. Note that the default *ensure_ascii* setting has much
   better performance in Python 2.

   The other options have the same meaning as in :func:`dump`.

.. function:: load(fp, encoding='utf-8', cls=None, object_hook=None, \
                   parse_float=None, parse_int=None, \
                   parse_constant=None, object_pairs_hook=None, \
                   use_decimal=None, **kw)

   Deserialize *fp* (a ``.read()``-supporting file-like object containing a JSON
   document) to a Python object using this
   :ref:`conversion table <json-to-py-table>`. :exc:`JSONDecodeError` will be
   raised if the given JSON document is not valid.

   If the contents of *fp* are encoded with an ASCII based encoding other than
   UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be specified.
   Encodings that are not ASCII based (such as UCS-2) are not allowed, and
   should be wrapped with ``codecs.getreader(fp)(encoding)``, or simply decoded
   to a :class:`unicode` object and passed to :func:`loads`. The default
   setting of ``'utf-8'`` is fastest and should be using whenever possible.

   If *fp.read()* returns :class:`str` then decoded JSON strings that contain
   only ASCII characters may be parsed as :class:`str` for performance and
   memory reasons. If your code expects only :class:`unicode` the appropriate
   solution is to wrap fp with a reader as demonstrated above.

   *object_hook* is an optional function that will be called with the result of
   any object literal decode (a :class:`dict`).  The return value of
   *object_hook* will be used instead of the :class:`dict`.  This feature can be used
   to implement custom decoders (e.g. `JSON-RPC <http://www.jsonrpc.org>`_
   class hinting).

   *object_pairs_hook* is an optional function that will be called with the
   result of any object literal decode with an ordered list of pairs.  The
   return value of *object_pairs_hook* will be used instead of the
   :class:`dict`.  This feature can be used to implement custom decoders that
   rely on the order that the key and value pairs are decoded (for example,
   :class:`collections.OrderedDict` will remember the order of insertion). If
   *object_hook* is also defined, the *object_pairs_hook* takes priority.

   .. versionchanged:: 2.1.0
      Added support for *object_pairs_hook*.

   *parse_float*, if specified, will be called with the string of every JSON
   float to be decoded.  By default, this is equivalent to ``float(num_str)``.
   This can be used to use another datatype or parser for JSON floats
   (e.g. :class:`decimal.Decimal`).

   *parse_int*, if specified, will be called with the string of every JSON int
   to be decoded.  By default, this is equivalent to ``int(num_str)``.  This can
   be used to use another datatype or parser for JSON integers
   (e.g. :class:`float`).

   *parse_constant*, if specified, will be called with one of the following
   strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.  This can be used to
   raise an exception if invalid JSON numbers are encountered.

   If *use_decimal* is true (default: ``False``) then *parse_float* is set to
   :class:`decimal.Decimal`. This is a convenience for parity with the
   :func:`dump` parameter.

   .. versionchanged:: 2.1.0
      *use_decimal* is new in 2.1.0.

   If *iterable_as_array* is true (default: ``False``),
   any object not in the above table that implements ``__iter__()``
   will be encoded as a JSON array.

   .. versionchanged:: 3.8.0
     *iterable_as_array* is new in 3.8.0.


   To use a custom :class:`JSONDecoder` subclass, specify it with the ``cls``
   kwarg.  Additional keyword arguments will be passed to the constructor of the
   class. You probably shouldn't do this.

    .. note::

        Subclassing is not recommended. You should use *object_hook* or
        *object_pairs_hook*. This is faster and more portable than subclassing.

    .. note::

        :func:`load` will read the rest of the file-like object as a string and
        then call :func:`loads`. It does not stop at the end of the first valid
        JSON document it finds and it will raise an error if there is anything
        other than whitespace after the document. Except for files containing
        only one JSON document, it is recommended to use :func:`loads`.

.. function:: loads(s, encoding='utf-8', cls=None, object_hook=None, \
                    parse_float=None, parse_int=None, \
                    parse_constant=None, object_pairs_hook=None, \
                    use_decimal=None, **kw)

   Deserialize *s* (a :class:`str` or :class:`unicode` instance containing a JSON
   document) to a Python object. :exc:`JSONDecodeError` will be
   raised if the given JSON document is not valid.

   If *s* is a :class:`str` instance and is encoded with an ASCII based encoding
   other than UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be
   specified.  Encodings that are not ASCII based (such as UCS-2) are not
   allowed and should be decoded to :class:`unicode` first.

   If *s* is a :class:`str` then decoded JSON strings that contain
   only ASCII characters may be parsed as :class:`str` for performance and
   memory reasons. If your code expects only :class:`unicode` the appropriate
   solution is decode *s* to :class:`unicode` prior to calling loads.

   The other arguments have the same meaning as in :func:`load`.

Encoders and decoders

Exceptions

.. exception:: JSONDecodeError(msg, doc, pos, end=None)

    Subclass of :exc:`ValueError` with the following additional attributes:

    .. attribute:: msg

        The unformatted error message

    .. attribute:: doc

        The JSON document being parsed

    .. attribute:: pos

        The start index of doc where parsing failed

    .. attribute:: end

        The end index of doc where parsing failed (may be ``None``)

    .. attribute:: lineno

        The line corresponding to pos

    .. attribute:: colno

        The column corresponding to pos

    .. attribute:: endlineno

        The line corresponding to end (may be ``None``)

    .. attribute:: endcolno

        The column corresponding to end (may be ``None``)

Standard Compliance and Interoperability

The JSON format is specified by RFC 7159 and by ECMA-404. This section details this module's level of compliance with the RFC. For simplicity, :class:`JSONEncoder` and :class:`JSONDecoder` subclasses, and parameters other than those explicitly mentioned, are not considered.

This module does not comply with the RFC in a strict fashion, implementing some extensions that are valid JavaScript but not valid JSON. In particular:

  • Infinite and NaN number values are accepted and output;
  • Repeated names within an object are accepted, and only the value of the last name-value pair is used.

Since the RFC permits RFC-compliant parsers to accept input texts that are not RFC-compliant, this module's deserializer is technically RFC-compliant under default settings.

Character Encodings

The RFC recommends that JSON be represented using either UTF-8, UTF-16, or UTF-32, with UTF-8 being the recommended default for maximum interoperability.

As permitted, though not required, by the RFC, this module's serializer sets ensure_ascii=True by default, thus escaping the output so that the resulting strings only contain ASCII characters.

Other than the ensure_ascii parameter, this module is defined strictly in terms of conversion between Python objects and :class:`Unicode strings <str>`, and thus does not otherwise directly address the issue of character encodings.

The RFC prohibits adding a byte order mark (BOM) to the start of a JSON text, and this module's serializer does not add a BOM to its output. The RFC permits, but does not require, JSON deserializers to ignore an initial BOM in their input. This module's deserializer will ignore an initial BOM, if present.

.. versionchanged:: 3.6.0
  Older versions would raise :exc:`ValueError` when an initial BOM is present

The RFC does not explicitly forbid JSON strings which contain byte sequences that don't correspond to valid Unicode characters (e.g. unpaired UTF-16 surrogates), but it does note that they may cause interoperability problems. By default, this module accepts and outputs (when present in the original :class:`str`) codepoints for such sequences.

Infinite and NaN Number Values

The RFC does not permit the representation of infinite or NaN number values. Despite that, by default, this module accepts and outputs Infinity, -Infinity, and NaN as if they were valid JSON number literal values:

>>> # Neither of these calls raises an exception, but the results are not valid JSON
>>> json.dumps(float('-inf'))
'-Infinity'
>>> json.dumps(float('nan'))
'NaN'
>>> # Same when deserializing
>>> json.loads('-Infinity')
-inf
>>> json.loads('NaN')
nan

In the serializer, the allow_nan parameter can be used to alter this behavior. In the deserializer, the parse_constant parameter can be used to alter this behavior.

Repeated Names Within an Object

The RFC specifies that the names within a JSON object should be unique, but does not mandate how repeated names in JSON objects should be handled. By default, this module does not raise an exception; instead, it ignores all but the last name-value pair for a given name:

>>> weird_json = '{"x": 1, "x": 2, "x": 3}'
>>> json.loads(weird_json) == {'x': 3}
True

The object_pairs_hook parameter can be used to alter this behavior.

Top-level Non-Object, Non-Array Values

The old version of JSON specified by the obsolete RFC 4627 required that the top-level value of a JSON text must be either a JSON object or array (Python :class:`dict` or :class:`list`), and could not be a JSON null, boolean, number, or string value. RFC 7159 removed that restriction, and this module does not and has never implemented that restriction in either its serializer or its deserializer.

Regardless, for maximum interoperability, you may wish to voluntarily adhere to the restriction yourself.

Implementation Limitations

Some JSON deserializer implementations may set limits on:

  • the size of accepted JSON texts
  • the maximum level of nesting of JSON objects and arrays
  • the range and precision of JSON numbers
  • the content and maximum length of JSON strings

This module does not impose any such limits beyond those of the relevant Python datatypes themselves or the Python interpreter itself.

When serializing to JSON, beware any such limitations in applications that may consume your JSON. In particular, it is common for JSON numbers to be deserialized into IEEE 754 double precision numbers and thus subject to that representation's range and precision limitations. This is especially relevant when serializing Python :class:`int` values of extremely large magnitude, or when serializing instances of "exotic" numerical types such as :class:`decimal.Decimal`.

Command Line Interface

The :mod:`simplejson.tool` module provides a simple command line interface to validate and pretty-print JSON.

If the optional :option:`infile` and :option:`outfile` arguments are not specified, :attr:`sys.stdin` and :attr:`sys.stdout` will be used respectively:

$ echo '{"json": "obj"}' | python -m simplejson.tool
{
    "json": "obj"
}
$ echo '{1.2:3.4}' | python -m simplejson.tool
Expecting property name enclosed in double quotes: line 1 column 2 (char 1)

Command line options

.. cmdoption:: infile

   The JSON file to be validated or pretty-printed::

      $ python -m simplejson.tool mp_films.json
      [
          {
              "title": "And Now for Something Completely Different",
              "year": 1971
          },
          {
              "title": "Monty Python and the Holy Grail",
              "year": 1975
          }
      ]

   If *infile* is not specified, read from :attr:`sys.stdin`.
.. cmdoption:: outfile

   Write the output of the *infile* to the given *outfile*. Otherwise, write it
   to :attr:`sys.stdout`.

Footnotes

[1]As noted in the errata for RFC 7159, JSON permits literal U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) characters in strings, whereas JavaScript (as of ECMAScript Edition 5.1) does not.