| :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 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 :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 (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) |
| |
| |
| 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. |