: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), 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] ).
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.
: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({'6': 7, '4': 5}, sort_keys=True, indent=4))
{
"4": 5,
"6": 7
}
Specializing 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']
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 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:
$ 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 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.
.. 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 characters escaped.
If ``False``, these characters will be outputted as-is.
: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), the most compact representation is used.
: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``.
.. 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`.
.. 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* 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 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 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 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.
.. function:: loads(s, *, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_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.
Simple JSON decoder.
Performs the following translations in decoding by default:
| 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 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*.
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.
Extensible JSON encoder for Python data structures.
Supports the following objects and types by default:
| Python | JSON |
|---|---|
| dict | 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.
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 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 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)
.. 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
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.
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.
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.
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.
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.
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`.
.. program:: json
.. 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:
$ 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.
.. 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.
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. |