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

.. module:: json :synopsis: Encode and decode the JSON format.

.. moduleauthor:: Bob Ippolito [email protected] .. sectionauthor:: Bob Ippolito [email protected]

Source code: :source:Lib/json/__init__.py

JSON (JavaScript Object Notation) <http://json.org>, specified by :rfc:7159 (which obsoletes :rfc:4627) and by ECMA-404 <http://www.ecma-international.org/publications/standards/Ecma-404.htm>, is a lightweight data interchange format inspired by JavaScript <https://en.wikipedia.org/wiki/JavaScript>_ object literal syntax (although it is not a strict subset of JavaScript [#rfc-errata]_ ).

:mod:json exposes an API familiar to users of the standard library :mod:marshal and :mod:pickle modules.

Encoding basic Python object hierarchies::

>>> import 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('\u1234'))
"\u1234"
>>> print(json.dumps('\\'))
"\\"
>>> print(json.dumps({"c": 0, "b": 0, "a": 0}, sort_keys=True))
{"a": 0, "b": 0, "c": 0}
>>> from io import StringIO
>>> io = StringIO()
>>> json.dump(['streaming API'], io)
>>> io.getvalue()
'["streaming API"]'

Compact encoding::

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

Pretty printing::

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

Decoding JSON::

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

Specializing JSON object decoding::

>>> import 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('1.1')

Extending :class:JSONEncoder::

>>> import json
>>> class ComplexEncoder(json.JSONEncoder):
...     def default(self, obj):
...         if isinstance(obj, complex):
...             return [obj.real, obj.imag]
...         # Let the base class default method raise the TypeError
...         return json.JSONEncoder.default(self, obj)
...
>>> json.dumps(2 + 1j, cls=ComplexEncoder)
'[2.0, 1.0]'
>>> ComplexEncoder().encode(2 + 1j)
'[2.0, 1.0]'
>>> list(ComplexEncoder().iterencode(2 + 1j))
['[2.0', ', 1.0', ']']

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

.. code-block:: shell-session

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

See :ref:json-commandline for detailed documentation.

.. note::

JSON is a subset of YAML <http://yaml.org/>_ 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, default=None,
sort_keys=False, **kw)

Serialize obj as a JSON formatted stream to fp (a .write()-supporting :term: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:int, :class:float, :class:bool, None) will be skipped instead of raising a :exc:TypeError.

The :mod:json module always produces :class:str objects, not :class:bytes objects. Therefore, fp.write() must support :class:str input.

If ensure_ascii is true (the default), the output is guaranteed to have all incoming non-ASCII characters escaped. If ensure_ascii is false, these characters will be output as-is.

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 JSON specification. If allow_nan is true, their JavaScript equivalents (NaN, Infinity, -Infinity) will be used.

If indent is a non-negative integer or string, then JSON array elements and object members will be pretty-printed with that indent level. An indent level of 0, negative, or "" will only insert newlines. None (the default) selects the most compact representation. Using a positive integer indent indents that many spaces per level. If indent is a string (such as "\t"), that string is used to indent each level.

.. versionchanged:: 3.2 Allow strings for indent in addition to integers.

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:: 3.4 Use (',', ': ') as default if indent is not None.

If specified, default should be a function that gets called for objects that can't otherwise be serialized. It should return a JSON encodable version of the object or raise a :exc:TypeError. If not specified, :exc:TypeError is raised.

If sort_keys is true (default: False), then the output of dictionaries will be sorted by key.

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; otherwise :class:JSONEncoder is used.

.. versionchanged:: 3.6 All optional parameters are now :ref:keyword-only <keyword-only_parameter>.

.. note::

  Unlike :mod:`pickle` and :mod:`marshal`, JSON is not a framed protocol,
  so trying to serialize multiple objects with repeated calls to
  :func:`dump` using the same *fp* will result in an invalid JSON file.

.. function:: dumps(obj, *, skipkeys=False, ensure_ascii=True,
check_circular=True, allow_nan=True, cls=None,
indent=None, separators=None, default=None,
sort_keys=False, **kw)

Serialize obj to a JSON formatted :class:str using this :ref:conversion table <py-to-json-table>. The arguments have the same meaning as in :func:dump.

.. note::

  Keys in key/value pairs of JSON are always of the type :class:`str`. When
  a dictionary is converted into JSON, all the keys of the dictionary are
  coerced to strings. As a result of this, if a dictionary is converted
  into JSON and then back into a dictionary, the dictionary may not equal
  the original one. That is, ``loads(dumps(x)) != x`` if x has non-string
  keys.

.. function:: load(fp, *, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)

Deserialize fp (a .read()-supporting :term:text file or :term:binary file containing a JSON document) to a Python object using this :ref:conversion table <json-to-py-table>.

object_hook is an optional function that will be called with the result of any object literal decoded (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 decoded 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. If object_hook is also defined, the object_pairs_hook takes priority.

.. versionchanged:: 3.1 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.

.. versionchanged:: 3.1 parse_constant doesn't get called on 'null', 'true', 'false' anymore.

To use a custom :class:JSONDecoder subclass, specify it with the cls kwarg; otherwise :class:JSONDecoder is used. Additional keyword arguments will be passed to the constructor of the class.

If the data being deserialized is not a valid JSON document, a :exc:JSONDecodeError will be raised.

.. versionchanged:: 3.6 All optional parameters are now :ref:keyword-only <keyword-only_parameter>.

.. versionchanged:: 3.6 fp can now be a :term:binary file. The input encoding should be UTF-8, UTF-16 or UTF-32.

.. function:: loads(s, *, encoding=None, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)

Deserialize s (a :class:str, :class:bytes or :class:bytearray instance containing a JSON document) to a Python object using this :ref:conversion table <json-to-py-table>.

The other arguments have the same meaning as in :func:load, except encoding which is ignored and deprecated.

If the data being deserialized is not a valid JSON document, a :exc:JSONDecodeError will be raised.

.. versionchanged:: 3.6 s can now be of type :class:bytes or :class:bytearray. The input encoding should be UTF-8, UTF-16 or UTF-32.

Encoders and Decoders

.. class:: JSONDecoder(*, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, strict=True, object_pairs_hook=None)

Simple JSON decoder.

Performs the following translations in decoding by default:

.. _json-to-py-table:

+---------------+-------------------+ | JSON | Python | +===============+===================+ | object | dict | +---------------+-------------------+ | array | list | +---------------+-------------------+ | string | str | +---------------+-------------------+ | number (int) | int | +---------------+-------------------+ | number (real) | float | +---------------+-------------------+ | true | True | +---------------+-------------------+ | false | False | +---------------+-------------------+ | null | None | +---------------+-------------------+

It also understands NaN, Infinity, and -Infinity as their corresponding float values, which is outside the JSON spec.

object_hook, if specified, will be called with the result of every JSON object decoded and its return value will be used in place of the given :class:dict. This can be used to provide custom deserializations (e.g. to support JSON-RPC class hinting).

object_pairs_hook, if specified will be called with the result of every JSON object decoded 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. If object_hook is also defined, the object_pairs_hook takes priority.

.. versionchanged:: 3.1 Added support for object_pairs_hook.

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 strict is false (True is the default), then control characters will be allowed inside strings. Control characters in this context are those with character codes in the 0--31 range, including '\t' (tab), '\n', '\r' and '\0'.

If the data being deserialized is not a valid JSON document, a :exc:JSONDecodeError will be raised.

.. versionchanged:: 3.6 All parameters are now :ref:keyword-only <keyword-only_parameter>.

.. method:: decode(s)

  Return the Python representation of *s* (a :class:`str` instance
  containing a JSON document).

  :exc:`JSONDecodeError` will be raised if the given JSON document is not
  valid.

.. method:: raw_decode(s)

  Decode a JSON document from *s* (a :class:`str` beginning with a
  JSON document) and return a 2-tuple of the Python representation
  and the index in *s* where the document ended.

  This can be used to decode a JSON document from a string that may have
  extraneous data at the end.

.. class:: JSONEncoder(*, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, sort_keys=False, indent=None, separators=None, default=None)

Extensible JSON encoder for Python data structures.

Supports the following objects and types by default:

.. _py-to-json-table:

.. versionchanged:: 3.4 Added support for int- and float-derived Enum classes.

To extend this to recognize other objects, subclass and implement a :meth:default method with another method that returns a serializable object for o if possible, otherwise it should call the superclass implementation (to raise :exc:TypeError).

If skipkeys is false (the default), then it is a :exc:TypeError to attempt encoding of keys that are not :class:str, :class:int, :class:float or None. If skipkeys is true, such items are simply skipped.

If ensure_ascii is true (the default), the output is guaranteed to have all incoming non-ASCII characters escaped. If ensure_ascii is false, these characters will be output as-is.

If check_circular is true (the default), then lists, dicts, and custom encoded objects will be checked for circular references during encoding to prevent an infinite recursion (which would cause an :exc:OverflowError). Otherwise, no such check takes place.

If allow_nan is true (the default), then NaN, Infinity, and -Infinity will be encoded as such. This behavior is not JSON specification compliant, but is consistent with most JavaScript based encoders and decoders. Otherwise, it will be a :exc:ValueError to encode such floats.

If sort_keys is true (default: False), 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:: 3.2 Allow strings for indent in addition to integers.

.. versionchanged:: 3.4 Use (',', ': ') as default if indent is not None.

.. versionchanged:: 3.6 All parameters are now :ref:keyword-only <keyword-only_parameter>.

.. method:: default(o)

  Implement this method in a subclass such that it returns a serializable
  object for *o*, or calls the base implementation (to raise a
  :exc:`TypeError`).

  For example, to support arbitrary iterators, you could implement default
  like this::

     def default(self, o):
        try:
            iterable = iter(o)
        except TypeError:
            pass
        else:
            return list(iterable)
        # Let the base class default method raise the TypeError
        return json.JSONEncoder.default(self, o)

.. method:: encode(o)

  Return a JSON string representation of a Python data structure, *o*.  For
  example::

    >>> json.JSONEncoder().encode({"foo": ["bar", "baz"]})
    '{"foo": ["bar", "baz"]}'

.. method:: iterencode(o)

  Encode the given object, *o*, and yield each string representation as
  available.  For example::

        for chunk in json.JSONEncoder().iterencode(bigobject):
            mysocket.write(chunk)

Exceptions

.. exception:: JSONDecodeError(msg, doc, pos)

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:: lineno

  The line corresponding to *pos*.

.. attribute:: colno

  The column corresponding to *pos*.

.. versionadded:: 3.5

Standard Compliance and Interoperability

The JSON format is specified by :rfc:7159 and by ECMA-404 <http://www.ecma-international.org/publications/standards/Ecma-404.htm>_. This section details this module's level of compliance with the RFC. For simplicity, :class:JSONEncoder and :class:JSONDecoder subclasses, and parameters other than those explicitly mentioned, are not considered.

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

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

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

Character Encodings ^^^^^^^^^^^^^^^^^^^

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

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 raises a :exc:ValueError when an initial BOM is present.

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

Infinite and NaN Number Values ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

Neither of these calls raises an exception, but the results are not valid JSON

json.dumps(float('-inf')) '-Infinity' json.dumps(float('nan')) 'NaN'

Same when deserializing

json.loads('-Infinity') -inf json.loads('NaN') nan

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

Repeated Names Within an Object ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

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

The object_pairs_hook parameter can be used to alter this behavior.

Top-level Non-Object, Non-Array Values ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

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

Implementation Limitations ^^^^^^^^^^^^^^^^^^^^^^^^^^

Some JSON deserializer implementations may set limits on:

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

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

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

.. _json-commandline: .. program:: json.tool

Command Line Interface

.. module:: json.tool :synopsis: A command line to validate and pretty-print JSON.

Source code: :source:Lib/json/tool.py

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

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

.. code-block:: shell-session

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

.. versionchanged:: 3.5 The output is now in the same order as the input. Use the :option:--sort-keys option to sort the output of dictionaries alphabetically by key.

Command line options ^^^^^^^^^^^^^^^^^^^^

.. cmdoption:: infile

The JSON file to be validated or pretty-printed:

.. code-block:: shell-session

  $ python -m json.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.

.. cmdoption:: --sort-keys

Sort the output of dictionaries alphabetically by key.

.. versionadded:: 3.5

.. cmdoption:: -h, --help

Show the help message.

.. rubric:: Footnotes

.. [#rfc-errata] As noted in the errata for RFC 7159 <https://www.rfc-editor.org/errata_search.php?rfc=7159>_, JSON permits literal U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) characters in strings, whereas JavaScript (as of ECMAScript Edition 5.1) does not.

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

:mod:json --- JSON encoder and decoder

Basic Usage

Encoders and Decoders

Exceptions

Standard Compliance and Interoperability

Neither of these calls raises an exception, but the results are not valid JSON

Same when deserializing

Command Line Interface

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