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:mod:`json` --- JSON encoder and decoder
========================================
.. module:: json
:synopsis: Encode and decode the JSON format.
.. moduleauthor:: Bob Ippolito <bob@redivi.com>
.. sectionauthor:: Bob Ippolito <bob@redivi.com>
`JSON (JavaScript Object Notation) <http://json.org>`_ is a subset of JavaScript
syntax (ECMA-262 3rd edition) used as a lightweight data interchange format.
: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]
... 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', ']']
.. highlight:: bash
Using json.tool from the shell to validate and pretty-print::
$ echo '{"json":"obj"}' | python -mjson.tool
{
"json": "obj"
}
$ echo '{1.2:3.4}' | python -mjson.tool
Expecting property name: line 1 column 1 (char 1)
.. highlight:: python3
.. note::
The JSON produced by this module's default settings is a subset of
YAML, so it may be used as a serializer for that as well.
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, **kw)
Serialize *obj* as a JSON formatted stream to *fp* (a ``.write()``-supporting
file-like object).
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, instead of
using the JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``).
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 at '\t'),
that string is used to indent each level.
If *separators* is an ``(item_separator, dict_separator)`` tuple, then it
will be used instead of the default ``(', ', ': ')`` separators. ``(',',
':')`` is the most compact JSON representation.
*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; otherwise :class:`JSONEncoder` is used.
.. function:: dumps(obj, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, **kw)
Serialize *obj* to a JSON formatted :class:`str`. The arguments have the
same meaning as in :func:`dump`.
.. 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.
.. 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 convered
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 file-like object containing a JSON
document) to a Python object.
*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 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 that
rely on the order that the key and value pairs are decoded (for example,
:func:`collections.OrderedDict` will remember the order of insertion). 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.
.. 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` instance containing a JSON document) to a
Python object.
The other arguments have the same meaning as in :func:`load`, except
*encoding* which is ignored and deprecated.
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 | 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 that rely on the order
that the key and value pairs are decoded (for example,
:func:`collections.OrderedDict` will remember the order of insertion). 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'``, ``'null'``, ``'true'``,
``'false'``. 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'``.
.. method:: decode(s)
Return the Python representation of *s* (a :class:`str` instance
containing a JSON document)
.. 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:
+-------------------+---------------+
| Python | JSON |
+===================+===============+
| dict | object |
+-------------------+---------------+
| list, tuple | array |
+-------------------+---------------+
| str | string |
+-------------------+---------------+
| int, float | number |
+-------------------+---------------+
| True | true |
+-------------------+---------------+
| False | false |
+-------------------+---------------+
| None | null |
+-------------------+---------------+
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 str, int, 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.
If *indent* is a non-negative integer (it is ``None`` by default), then JSON
array elements and object members will be pretty-printed with that indent
level. An indent level of 0 will only insert newlines. ``None`` is the most
compact representation.
If specified, *separators* should be an ``(item_separator, key_separator)``
tuple. The default is ``(', ', ': ')``. To get the most compact JSON
representation, you should specify ``(',', ':')`` to eliminate whitespace.
If specified, *default* is 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`.
.. 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)
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)
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