Mirror
/
cpython
mirror of https://github.com/python/cpython
4
1
Fork 0
Code Issues Releases Wiki Activity
cpython/Doc/library/json.rst

566 lines
22 KiB
ReStructuredText
Raw Blame History

: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>
.. versionadded:: 2.6
`JSON (JavaScript Object Notation) <http://json.org>`_, specified by
:rfc:`4627`, is a lightweight data interchange format based on a subset of
`JavaScript <http://en.wikipedia.org/wiki/JavaScript>`_ syntax (`ECMA-262 3rd
edition <http://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%203rd%20edition,%20December%201999.pdf>`_).
: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(u'\u1234')
"\u1234"
>>> print json.dumps('\\')
"\\"
>>> print json.dumps({"c": 0, "b": 0, "a": 0}, sort_keys=True)
{"a": 0, "b": 0, "c": 0}
>>> from StringIO 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]}]')
[u'foo', {u'bar': [u'baz', None, 1.0, 2]}]
>>> json.loads('"\\"foo\\bar"')
u'"foo\x08ar'
>>> from StringIO import StringIO
>>> io = StringIO('["streaming API"]')
>>> json.load(io)
[u'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)
...
>>> 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:: none
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 enclosed in double quotes: line 1 column 1 (char 1)
.. highlight:: python
.. 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, encoding="utf-8", \
default=None, sort_keys=False, **kw)
Serialize *obj* as a JSON formatted stream to *fp* (a ``.write()``-supporting
:term:`file-like object`).
If *skipkeys* is ``True`` (default: ``False``), then dict keys that are not
of a basic type (:class:`str`, :class:`unicode`, :class:`int`, :class:`long`,
:class:`float`, :class:`bool`, ``None``) will be skipped instead of raising a
:exc:`TypeError`.
If *ensure_ascii* is ``True`` (the default), all non-ASCII characters in the
output are escaped with ``\uXXXX`` sequences, and the result is a
:class:`str` instance consisting of ASCII characters only. If
*ensure_ascii* is ``False``, some chunks written to *fp* may be
:class:`unicode` instances. This usually happens because the input contains
unicode strings or the *encoding* parameter is used. Unless ``fp.write()``
explicitly understands :class:`unicode` (as in :func:`codecs.getwriter`)
this is likely to cause an error.
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, then JSON array elements and object
members will be pretty-printed with that indent level. An indent level of 0,
or negative, will only insert newlines. ``None`` (the default) selects the
most compact representation.
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.
*encoding* is the character encoding for str instances, default is UTF-8.
*default(obj)* is a function that should return a serializable version of
*obj* or raise :exc:`TypeError`. The default simply raises :exc:`TypeError`.
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.
.. note::
Unlike :mod:`pickle` and :mod:`marshal`, JSON is not a framed protocol so
trying to serialize more objects with repeated calls to :func:`dump` and
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, encoding="utf-8", \
default=None, sort_keys=False, **kw)
Serialize *obj* to a JSON formatted :class:`str`. If *ensure_ascii* is
``False``, the result may contain non-ASCII characters and the return value
may be a :class:`unicode` instance.
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 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[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[, **kw]]]]]]]])
Deserialize *fp* (a ``.read()``-supporting :term:`file-like object`
containing a JSON document) to a Python object.
If the contents of *fp* are encoded with an ASCII based encoding other than
UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be specified.
Encodings that are not ASCII based (such as UCS-2) are not allowed, and
should be wrapped with ``codecs.getreader(encoding)(fp)``, or simply decoded
to a :class:`unicode` object and passed to :func:`loads`.
*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 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:: 2.7
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:: 2.7
*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[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[, **kw]]]]]]]])
Deserialize *s* (a :class:`str` or :class:`unicode` instance containing a JSON
document) to a Python object.
If *s* is a :class:`str` instance and is encoded with an ASCII based encoding
other than UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be
specified. Encodings that are not ASCII based (such as UCS-2) are not
allowed and should be decoded to :class:`unicode` first.
The other arguments have the same meaning as in :func:`load`.
Encoders and Decoders
---------------------
.. class:: JSONDecoder([encoding[, object_hook[, parse_float[, parse_int[, parse_constant[, strict[, object_pairs_hook]]]]]]])
Simple JSON decoder.
Performs the following translations in decoding by default:
+---------------+-------------------+
| JSON | Python |
+===============+===================+
| object | dict |
+---------------+-------------------+
| array | list |
+---------------+-------------------+
| string | unicode |
+---------------+-------------------+
| number (int) | int, long |
+---------------+-------------------+
| 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.
*encoding* determines the encoding used to interpret any :class:`str` objects
decoded by this instance (UTF-8 by default). It has no effect when decoding
:class:`unicode` objects.
Note that currently only encodings that are a superset of ASCII work, strings
of other encodings should be passed in as :class:`unicode`.
*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:: 2.7
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` or
:class:`unicode` instance containing a JSON document)
.. method:: raw_decode(s)
Decode a JSON document from *s* (a :class:`str` or :class:`unicode`
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[, ensure_ascii[, check_circular[, allow_nan[, sort_keys[, indent[, separators[, encoding[, default]]]]]]]]])
Extensible JSON encoder for Python data structures.
Supports the following objects and types by default:
+-------------------+---------------+
| Python | JSON |
+===================+===============+
| dict | object |
+-------------------+---------------+
| list, tuple | array |
+-------------------+---------------+
| str, unicode | string |
+-------------------+---------------+
| int, long, 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, long, float or None. If
*skipkeys* is ``True``, such items are simply skipped.
If *ensure_ascii* is ``True`` (the default), all non-ASCII characters in the
output are escaped with ``\uXXXX`` sequences, and the results are
:class:`str` instances consisting of ASCII characters only. If
*ensure_ascii* is ``False``, a result may be a :class:`unicode`
instance. This usually happens if the input contains unicode strings or the
*encoding* parameter is used.
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`.
If *encoding* is not ``None``, then all input strings will be transformed
into unicode using that encoding prior to JSON-encoding. The default is
UTF-8.
.. 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 JSONEncoder.default(self, o)
.. method:: encode(o)
Return a JSON string representation of a Python data structure, *o*. For
example::
>>> 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 JSONEncoder().iterencode(bigobject):
mysocket.write(chunk)
Standard Compliance
-------------------
The JSON format is specified by :rfc:`4627`. 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:
- Top-level non-object, non-array values are accepted and output;
- Infinite and NaN number values are accepted and output;
- Repeated names within an object are accepted, and only the value of the last
name-value pair is used.
Since the RFC permits RFC-compliant parsers to accept input texts that are not
RFC-compliant, this module's deserializer is technically RFC-compliant under
default settings.
Character Encodings
^^^^^^^^^^^^^^^^^^^
The RFC recommends that JSON be represented using either UTF-8, UTF-16, or
UTF-32, with UTF-8 being the default. Accordingly, this module uses UTF-8 as
the default for its *encoding* parameter.
This module's deserializer only directly works with ASCII-compatible encodings;
UTF-16, UTF-32, and other ASCII-incompatible encodings require the use of
workarounds described in the documentation for the deserializer's *encoding*
parameter.
The RFC also non-normatively describes a limited encoding detection technique
for JSON texts; this module's deserializer does not implement this or any other
kind of encoding detection.
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.
Top-level Non-Object, Non-Array Values
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The RFC specifies that the top-level value of a JSON text must be either a
JSON object or array (Python :class:`dict` or :class:`list`). This module's
deserializer also accepts input texts consisting solely of a
JSON null, boolean, number, or string value::
>>> just_a_json_string = '"spam and eggs"' # Not by itself a valid JSON text
>>> json.loads(just_a_json_string)
u'spam and eggs'
This module itself does not include a way to request that such input texts be
regarded as illegal. Likewise, this module's serializer also accepts single
Python :data:`None`, :class:`bool`, numeric, and :class:`str`
values as input and will generate output texts consisting solely of a top-level
JSON null, boolean, number, or string value without raising an exception::
>>> neither_a_list_nor_a_dict = u"spam and eggs"
>>> json.dumps(neither_a_list_nor_a_dict) # The result is not a valid JSON text
'"spam and eggs"'
This module's serializer does not itself include a way to enforce the
aforementioned constraint.
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 specify 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)
{u'x': 3}
The *object_pairs_hook* parameter can be used to alter this behavior.
Reference in New Issue View Git Blame Copy Permalink