Doc/library/json.rst

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

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

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

--------------

`JSON (JavaScript Object Notation) <https://json.org>`_, specified by
:rfc:`7159` (which obsoletes :rfc:`4627`) and by
`ECMA-404 <https://ecma-international.org/publications-and-standards/standards/ecma-404/>`_,
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]_ ).

.. note::
   The term "object" in the context of JSON processing in Python can be
   ambiguous. All values in Python are objects. In JSON, an object refers to
   any data wrapped in curly braces, similar to a Python dictionary.

.. warning::
   Be cautious when parsing JSON data from untrusted sources. A malicious
   JSON string may cause the decoder to consume considerable CPU and memory
   resources. Limiting the size of data to be parsed is recommended.

This module 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({'6': 7, '4': 5}, sort_keys=True, indent=4))
    {
        "4": 5,
        "6": 7
    }

Customizing JSON object encoding::

   >>> import json
   >>> def custom_json(obj):
   ...     if isinstance(obj, complex):
   ...         return {'__complex__': True, 'real': obj.real, 'imag': obj.imag}
   ...     raise TypeError(f'Cannot serialize object of {type(obj)}')
   ...
   >>> json.dumps(1 + 2j, default=custom_json)
   '{"__complex__": true, "real": 1.0, "imag": 2.0}'

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']

Customizing 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 super().default(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` from the shell to validate and pretty-print:

.. code-block:: shell-session

    $ echo '{"json":"obj"}' | python -m json
    {
        "json": "obj"
    }
    $ echo '{1.2:3.4}' | python -m json
    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 <https://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.

.. note::

   This module's encoders and decoders preserve input and output order by
   default.  Order is only lost if the underlying containers are unordered.


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:`Python-to-JSON conversion table
   <py-to-json-table>`.

   .. 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.

   :param object obj:
      The Python object to be serialized.

   :param fp:
      The file-like object *obj* will be serialized to.
      The :mod:`!json` module always produces :class:`str` objects,
      not :class:`bytes` objects,
      therefore ``fp.write()`` must support :class:`str` input.
   :type fp: :term:`file-like object`

   :param bool skipkeys:
      If ``True``, 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`.
      Default ``False``.

   :param bool ensure_ascii:
      If ``True`` (the default), the output is guaranteed to
      have all incoming non-ASCII and non-printable characters escaped.
      If ``False``, all characters will be outputted as-is, except for
      the characters that must be escaped: quotation mark, reverse solidus,
      and the control characters U+0000 through U+001F.

   :param bool check_circular:
      If ``False``, the circular reference check for container types is skipped
      and a circular reference will result in a :exc:`RecursionError` (or worse).
      Default ``True``.

   :param bool allow_nan:
      If ``False``, serialization of out-of-range :class:`float` values
      (``nan``, ``inf``, ``-inf``) will result in a :exc:`ValueError`,
      in strict compliance with the JSON specification.
      If ``True`` (the default), their JavaScript equivalents
      (``NaN``, ``Infinity``, ``-Infinity``) are used.

   :param cls:
      If set, a custom JSON encoder with the
      :meth:`~JSONEncoder.default` method overridden,
      for serializing into custom datatypes.
      If ``None`` (the default), :class:`!JSONEncoder` is used.
   :type cls: a :class:`JSONEncoder` subclass

   :param indent:
      If a positive integer or string, JSON array elements and
      object members will be pretty-printed with that indent level.
      A positive integer indents that many spaces per level;
      a string (such as ``"\t"``) is used to indent each level.
      If zero, negative, or ``""`` (the empty string),
      only newlines are inserted.
      If ``None`` (the default), no newlines are inserted.
   :type indent: int | str | None

   :param separators:
      A two-tuple: ``(item_separator, key_separator)``.
      If ``None`` (the default), *separators* defaults to
      ``(', ', ': ')`` if *indent* is ``None``,
      and ``(',', ': ')`` otherwise.
      For the most compact JSON,
      specify ``(',', ':')`` to eliminate whitespace.
   :type separators: tuple | None

   :param default:
      A function that is 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 ``None`` (the default), :exc:`!TypeError` is raised.
   :type default: :term:`callable` | None

   :param bool sort_keys:
      If ``True``, dictionaries will be outputted sorted by key.
      Default ``False``.

   .. 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
      converted 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. *sort_keys* sorts the keys before they are converted to strings,
      so numeric keys are sorted by value, not by their string representation.

   .. 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 optional parameters are now :ref:`keyword-only <keyword-only_parameter>`.


.. 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`.

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

   Deserialize *fp* to a Python object
   using the :ref:`JSON-to-Python conversion table <json-to-py-table>`.

   :param fp:
      A ``.read()``-supporting :term:`text file` or :term:`binary file`
      containing the JSON document to be deserialized.
   :type fp: :term:`file-like object`

   :param cls:
      If set, a custom JSON decoder.
      Additional keyword arguments to :func:`!load`
      will be passed to the constructor of *cls*.
      If ``None`` (the default), :class:`!JSONDecoder` is used.
   :type cls: a :class:`JSONDecoder` subclass

   :param object_hook:
      If set, a function that is called with the result of
      any JSON object literal decoded (a :class:`dict`).
      The return value of this function will be used
      instead of the :class:`dict`.
      This feature can be used to implement custom decoders,
      for example `JSON-RPC <https://www.jsonrpc.org>`_ class hinting.
      Default ``None``.
   :type object_hook: :term:`callable` | None

   :param object_pairs_hook:
      If set, a function that is called with the result of
      any JSON object literal decoded with an ordered list of pairs.
      The return value of this function will be used
      instead of the :class:`dict`.
      This feature can be used to implement custom decoders.
      If *object_hook* is also set, *object_pairs_hook* takes priority.
      Default ``None``.
   :type object_pairs_hook: :term:`callable` | None

   :param array_hook:
      If set, a function that is called with the result of
      any JSON array literal decoded with as a Python list.
      The return value of this function will be used
      instead of the :class:`list`.
      This feature can be used to implement custom decoders.
      Default ``None``.
   :type array_hook: :term:`callable` | None

   :param parse_float:
      If set, a function that is called with
      the string of every JSON float to be decoded.
      If ``None`` (the default), it is equivalent to ``float(num_str)``.
      This can be used to parse JSON floats into custom datatypes,
      for example :class:`decimal.Decimal`.
   :type parse_float: :term:`callable` | None

   :param parse_int:
      If set, a function that is called with
      the string of every JSON int to be decoded.
      If ``None`` (the default), it is equivalent to ``int(num_str)``.
      This can be used to parse JSON integers into custom datatypes,
      for example :class:`float`.
   :type parse_int: :term:`callable` | None

   :param parse_constant:
      If set, a function that is called with one of the following strings:
      ``'-Infinity'``, ``'Infinity'``, or ``'NaN'``.
      This can be used to raise an exception
      if invalid JSON numbers are encountered.
      Default ``None``.
   :type parse_constant: :term:`callable` | None

   :raises JSONDecodeError:
      When the data being deserialized is not a valid JSON document.

   :raises UnicodeDecodeError:
      When the data being deserialized does not contain
      UTF-8, UTF-16 or UTF-32 encoded data.

   .. versionchanged:: 3.1

      * Added the optional *object_pairs_hook* parameter.
      * *parse_constant* doesn't get called on 'null', 'true', 'false' anymore.

   .. versionchanged:: 3.6

      * All optional parameters are now :ref:`keyword-only <keyword-only_parameter>`.
      * *fp* can now be a :term:`binary file`.
        The input encoding should be UTF-8, UTF-16 or UTF-32.

   .. versionchanged:: 3.11
      The default *parse_int* of :func:`int` now limits the maximum length of
      the integer string via the interpreter's :ref:`integer string
      conversion length limitation <int_max_str_digits>` to help avoid denial
      of service attacks.

   .. versionchanged:: 3.15
      Added the optional *array_hook* parameter.

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

   Identical to :func:`load`, but instead of a file-like object,
   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>`.

   .. 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.

   .. versionchanged:: 3.9
      The keyword argument *encoding* has been removed.


Encoders and Decoders
---------------------

.. class:: JSONDecoder(*, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, strict=True, object_pairs_hook=None, array_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* is an optional function that 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 <https://www.jsonrpc.org>`_ class hinting).

   *object_pairs_hook* is an optional function that 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*.

   *array_hook* is an optional function that will be called with the
   result of every JSON array decoded as a list. The return value of
   *array_hook* will be used instead of the :class:`list`. This feature can be
   used to implement custom decoders.

   .. versionchanged:: 3.15
      Added support for *array_hook*.

   *parse_float* is an optional function that 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* is an optional function that 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* is an optional function that 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:

   +----------------------------------------+---------------+
   | Python                                 | JSON          |
   +========================================+===============+
   | dict, frozendict                       | object        |
   +----------------------------------------+---------------+
   | list, tuple                            | array         |
   +----------------------------------------+---------------+
   | str                                    | string        |
   +----------------------------------------+---------------+
   | int, float, int- & float-derived Enums | number        |
   +----------------------------------------+---------------+
   | True                                   | true          |
   +----------------------------------------+---------------+
   | False                                  | false         |
   +----------------------------------------+---------------+
   | None                                   | null          |
   +----------------------------------------+---------------+

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

   .. versionchanged:: 3.15
      Added support for :class:`frozendict`.

   To extend this to recognize other objects, subclass and implement a
   :meth:`~JSONEncoder.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), a :exc:`TypeError` will be raised when
   trying to encode keys that are not :class:`str`, :class:`int`, :class:`float`,
   :class:`bool` 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 and non-printable characters escaped.
   If *ensure_ascii* is false, all characters will be output as-is, except for
   the characters that must be escaped: quotation mark, reverse solidus,
   and the control characters U+0000 through U+001F.

   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 a :exc:`RecursionError`).
   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.

   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.

   .. 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
      :meth:`~JSONEncoder.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 super().default(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 <https://ecma-international.org/publications-and-standards/standards/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 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 printable 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

Command-line interface
----------------------

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

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

--------------

The :mod:`!json` module can be invoked as a script via ``python -m json``
to validate and pretty-print JSON objects. The :mod:`!json.tool` submodule
implements this interface.

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

.. code-block:: shell-session

    $ echo '{"json": "obj"}' | python -m json
    {
        "json": "obj"
    }
    $ echo '{1.2:3.4}' | python -m json
    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.

.. versionchanged:: 3.14
   The :mod:`!json` module may now be directly executed as
   ``python -m json``. For backwards compatibility, invoking
   the CLI as ``python -m json.tool`` remains supported.


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

.. option:: infile

   The JSON file to be validated or pretty-printed:

   .. code-block:: shell-session

      $ python -m json 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 :data:`sys.stdin`.

.. option:: outfile

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

.. option:: --sort-keys

   Sort the output of dictionaries alphabetically by key.

   .. versionadded:: 3.5

.. option:: --no-ensure-ascii

   Disable escaping of non-ascii characters, see :func:`json.dumps` for more information.

   .. versionadded:: 3.9

.. option:: --json-lines

   Parse every input line as separate JSON object.

   .. versionadded:: 3.8

.. option:: --indent, --tab, --no-indent, --compact

   Mutually exclusive options for whitespace control.

   .. versionadded:: 3.9

.. option:: -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.