531 lines
21 KiB
ReStructuredText
531 lines
21 KiB
ReStructuredText
: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>`_, 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('\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 enclosed in double quotes: line 1 column 1 (char 1)
|
|
|
|
.. highlight:: python3
|
|
|
|
.. note::
|
|
|
|
JSON is a subset of `YAML <http://yaml.org/>`_ 1.2. The JSON produced by
|
|
this module's default settings (in particular, the default *separators*
|
|
value) is also a subset of YAML 1.0 and 1.1. This module can thus also be
|
|
used as a YAML serializer.
|
|
|
|
|
|
Basic Usage
|
|
-----------
|
|
|
|
.. function:: dump(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, **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:`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 as ``"\t"``),
|
|
that string is used to indent each level.
|
|
|
|
.. versionchanged:: 3.2
|
|
Allow strings for *indent* in addition to integers.
|
|
|
|
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 :term:`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 <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:: 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 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 ``(', ', ': ')``. 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)
|
|
|
|
|
|
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.
|
|
|
|
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 address the issue
|
|
of character encodings.
|
|
|
|
|
|
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)
|
|
'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 = "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)
|
|
{'x': 3}
|
|
|
|
The *object_pairs_hook* parameter can be used to alter this behavior.
|