:mod:`json` --- JSON encoder and decoder ======================================== .. module:: json :synopsis: Encode and decode the JSON format. .. moduleauthor:: Bob Ippolito .. sectionauthor:: Bob Ippolito `JSON (JavaScript Object Notation) `_, specified by :rfc:`4627`, is a lightweight data interchange format based on a subset of `JavaScript `_ syntax (`ECMA-262 3rd edition `_). :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, separators=(',', ': '))) { "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] ... # Let the base class default method raise the TypeError ... 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 2 (char 1) .. highlight:: python3 .. 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. 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, \ 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:`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. .. note:: Since the default item separator is ``', '``, the output might include trailing whitespace when *indent* is specified. You can use ``separators=(',', ': ')`` to avoid this. 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`. 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. .. 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`. 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 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* (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 `_ 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. .. note:: Since the default item separator is ``', '``, the output might include trailing whitespace when *indent* is specified. You can use ``separators=(',', ': ')`` to avoid this. 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) # Let the base class default method raise the TypeError 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 `, 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.