/Doc/library/json.rst

  1:mod:`json` --- JSON encoder and decoder
  2========================================
  3
  4.. module:: json
  5   :synopsis: Encode and decode the JSON format.
  6.. moduleauthor:: Bob Ippolito <bob@redivi.com>
  7.. sectionauthor:: Bob Ippolito <bob@redivi.com>
  8.. versionadded:: 2.6
  9
 10JSON (JavaScript Object Notation) <http://json.org> is a subset of JavaScript
 11syntax (ECMA-262 3rd edition) used as a lightweight data interchange format.
 12
 13:mod:`json` exposes an API familiar to users of the standard library
 14:mod:`marshal` and :mod:`pickle` modules.
 15
 16Encoding basic Python object hierarchies::
 17
 18    >>> import json
 19    >>> json.dumps(['foo', {'bar': ('baz', None, 1.0, 2)}])
 20    '["foo", {"bar": ["baz", null, 1.0, 2]}]'
 21    >>> print json.dumps("\"foo\bar")
 22    "\"foo\bar"
 23    >>> print json.dumps(u'\u1234')
 24    "\u1234"
 25    >>> print json.dumps('\\')
 26    "\\"
 27    >>> print json.dumps({"c": 0, "b": 0, "a": 0}, sort_keys=True)
 28    {"a": 0, "b": 0, "c": 0}
 29    >>> from StringIO import StringIO
 30    >>> io = StringIO()
 31    >>> json.dump(['streaming API'], io)
 32    >>> io.getvalue()
 33    '["streaming API"]'
 34
 35Compact encoding::
 36
 37    >>> import json
 38    >>> json.dumps([1,2,3,{'4': 5, '6': 7}], separators=(',',':'))
 39    '[1,2,3,{"4":5,"6":7}]'
 40
 41Pretty printing::
 42
 43    >>> import json
 44    >>> print json.dumps({'4': 5, '6': 7}, sort_keys=True, indent=4)
 45    {
 46        "4": 5,
 47        "6": 7
 48    }
 49
 50Decoding JSON::
 51
 52    >>> import json
 53    >>> json.loads('["foo", {"bar":["baz", null, 1.0, 2]}]')
 54    [u'foo', {u'bar': [u'baz', None, 1.0, 2]}]
 55    >>> json.loads('"\\"foo\\bar"')
 56    u'"foo\x08ar'
 57    >>> from StringIO import StringIO
 58    >>> io = StringIO('["streaming API"]')
 59    >>> json.load(io)
 60    [u'streaming API']
 61
 62Specializing JSON object decoding::
 63
 64    >>> import json
 65    >>> def as_complex(dct):
 66    ...     if '__complex__' in dct:
 67    ...         return complex(dct['real'], dct['imag'])
 68    ...     return dct
 69    ...
 70    >>> json.loads('{"__complex__": true, "real": 1, "imag": 2}',
 71    ...     object_hook=as_complex)
 72    (1+2j)
 73    >>> import decimal
 74    >>> json.loads('1.1', parse_float=decimal.Decimal)
 75    Decimal('1.1')
 76
 77Extending :class:`JSONEncoder`::
 78
 79    >>> import json
 80    >>> class ComplexEncoder(json.JSONEncoder):
 81    ...     def default(self, obj):
 82    ...         if isinstance(obj, complex):
 83    ...             return [obj.real, obj.imag]
 84    ...         return json.JSONEncoder.default(self, obj)
 85    ...
 86    >>> dumps(2 + 1j, cls=ComplexEncoder)
 87    '[2.0, 1.0]'
 88    >>> ComplexEncoder().encode(2 + 1j)
 89    '[2.0, 1.0]'
 90    >>> list(ComplexEncoder().iterencode(2 + 1j))
 91    ['[', '2.0', ', ', '1.0', ']']
 92
 93
 94.. highlight:: none
 95
 96Using json.tool from the shell to validate and pretty-print::
 97
 98    $ echo '{"json":"obj"}' | python -mjson.tool
 99    {
100        "json": "obj"
101    }
102    $ echo '{ 1.2:3.4}' | python -mjson.tool
103    Expecting property name: line 1 column 2 (char 2)
104
105.. highlight:: python
106
107.. note::
108
109   The JSON produced by this module's default settings is a subset of
110   YAML, so it may be used as a serializer for that as well.
111
112
113Basic Usage
114-----------
115
116.. function:: dump(obj, fp[, skipkeys[, ensure_ascii[, check_circular[, allow_nan[, cls[, indent[, separators[, encoding[, default[, **kw]]]]]]]]]])
117
118   Serialize *obj* as a JSON formatted stream to *fp* (a ``.write()``-supporting
119   file-like object).
120
121   If *skipkeys* is ``True`` (default: ``False``), then dict keys that are not
122   of a basic type (:class:`str`, :class:`unicode`, :class:`int`, :class:`long`,
123   :class:`float`, :class:`bool`, ``None``) will be skipped instead of raising a
124   :exc:`TypeError`.
125
126   If *ensure_ascii* is ``False`` (default: ``True``), then some chunks written
127   to *fp* may be :class:`unicode` instances, subject to normal Python
128   :class:`str` to :class:`unicode` coercion rules.  Unless ``fp.write()``
129   explicitly understands :class:`unicode` (as in :func:`codecs.getwriter`) this
130   is likely to cause an error.
131
132   If *check_circular* is ``False`` (default: ``True``), then the circular
133   reference check for container types will be skipped and a circular reference
134   will result in an :exc:`OverflowError` (or worse).
135
136   If *allow_nan* is ``False`` (default: ``True``), then it will be a
137   :exc:`ValueError` to serialize out of range :class:`float` values (``nan``,
138   ``inf``, ``-inf``) in strict compliance of the JSON specification, instead of
139   using the JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``).
140
141   If *indent* is a non-negative integer, then JSON array elements and object
142   members will be pretty-printed with that indent level.  An indent level of 0
143   will only insert newlines.  ``None`` (the default) selects the most compact
144   representation.
145
146   If *separators* is an ``(item_separator, dict_separator)`` tuple, then it
147   will be used instead of the default ``(', ', ': ')`` separators.  ``(',',
148   ':')`` is the most compact JSON representation.
149
150   *encoding* is the character encoding for str instances, default is UTF-8.
151
152   *default(obj)* is a function that should return a serializable version of
153   *obj* or raise :exc:`TypeError`.  The default simply raises :exc:`TypeError`.
154
155   To use a custom :class:`JSONEncoder` subclass (e.g. one that overrides the
156   :meth:`default` method to serialize additional types), specify it with the
157   *cls* kwarg.
158
159
160.. function:: dumps(obj[, skipkeys[, ensure_ascii[, check_circular[, allow_nan[, cls[, indent[, separators[, encoding[, default[, **kw]]]]]]]]]])
161
162   Serialize *obj* to a JSON formatted :class:`str`.
163
164   If *ensure_ascii* is ``False``, then the return value will be a
165   :class:`unicode` instance.  The other arguments have the same meaning as in
166   :func:`dump`.
167
168
169.. function:: load(fp[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, **kw]]]]]]])
170
171   Deserialize *fp* (a ``.read()``-supporting file-like object containing a JSON
172   document) to a Python object.
173
174   If the contents of *fp* are encoded with an ASCII based encoding other than
175   UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be specified.
176   Encodings that are not ASCII based (such as UCS-2) are not allowed, and
177   should be wrapped with ``codecs.getreader(encoding)(fp)``, or simply decoded
178   to a :class:`unicode` object and passed to :func:`loads`.
179
180   *object_hook* is an optional function that will be called with the result of
181   any object literal decode (a :class:`dict`).  The return value of
182   *object_hook* will be used instead of the :class:`dict`.  This feature can be used
183   to implement custom decoders (e.g. JSON-RPC class hinting).
184
185   *parse_float*, if specified, will be called with the string of every JSON
186   float to be decoded.  By default, this is equivalent to ``float(num_str)``.
187   This can be used to use another datatype or parser for JSON floats
188   (e.g. :class:`decimal.Decimal`).
189
190   *parse_int*, if specified, will be called with the string of every JSON int
191   to be decoded.  By default, this is equivalent to ``int(num_str)``.  This can
192   be used to use another datatype or parser for JSON integers
193   (e.g. :class:`float`).
194
195   *parse_constant*, if specified, will be called with one of the following
196   strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``, ``'null'``, ``'true'``,
197   ``'false'``.  This can be used to raise an exception if invalid JSON numbers
198   are encountered.
199
200   To use a custom :class:`JSONDecoder` subclass, specify it with the ``cls``
201   kwarg.  Additional keyword arguments will be passed to the constructor of the
202   class.
203
204
205.. function:: loads(s[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, **kw]]]]]]])
206
207   Deserialize *s* (a :class:`str` or :class:`unicode` instance containing a JSON
208   document) to a Python object.
209
210   If *s* is a :class:`str` instance and is encoded with an ASCII based encoding
211   other than UTF-8 (e.g. latin-1), then an appropriate *encoding* name must be
212   specified.  Encodings that are not ASCII based (such as UCS-2) are not
213   allowed and should be decoded to :class:`unicode` first.
214
215   The other arguments have the same meaning as in :func:`dump`.
216
217
218Encoders and decoders
219---------------------
220
221.. class:: JSONDecoder([encoding[, object_hook[, parse_float[, parse_int[, parse_constant[, strict]]]]]])
222
223   Simple JSON decoder.
224
225   Performs the following translations in decoding by default:
226
227   +---------------+-------------------+
228   | JSON          | Python            |
229   +===============+===================+
230   | object        | dict              |
231   +---------------+-------------------+
232   | array         | list              |
233   +---------------+-------------------+
234   | string        | unicode           |
235   +---------------+-------------------+
236   | number (int)  | int, long         |
237   +---------------+-------------------+
238   | number (real) | float             |
239   +---------------+-------------------+
240   | true          | True              |
241   +---------------+-------------------+
242   | false         | False             |
243   +---------------+-------------------+
244   | null          | None              |
245   +---------------+-------------------+
246
247   It also understands ``NaN``, ``Infinity``, and ``-Infinity`` as their
248   corresponding ``float`` values, which is outside the JSON spec.
249
250   *encoding* determines the encoding used to interpret any :class:`str` objects
251   decoded by this instance (UTF-8 by default).  It has no effect when decoding
252   :class:`unicode` objects.
253
254   Note that currently only encodings that are a superset of ASCII work, strings
255   of other encodings should be passed in as :class:`unicode`.
256
257   *object_hook*, if specified, will be called with the result of every JSON
258   object decoded and its return value will be used in place of the given
259   :class:`dict`.  This can be used to provide custom deserializations (e.g. to
260   support JSON-RPC class hinting).
261
262   *parse_float*, if specified, will be called with the string of every JSON
263   float to be decoded.  By default, this is equivalent to ``float(num_str)``.
264   This can be used to use another datatype or parser for JSON floats
265   (e.g. :class:`decimal.Decimal`).
266
267   *parse_int*, if specified, will be called with the string of every JSON int
268   to be decoded.  By default, this is equivalent to ``int(num_str)``.  This can
269   be used to use another datatype or parser for JSON integers
270   (e.g. :class:`float`).
271
272   *parse_constant*, if specified, will be called with one of the following
273   strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``, ``'null'``, ``'true'``,
274   ``'false'``.  This can be used to raise an exception if invalid JSON numbers
275   are encountered.
276
277
278   .. method:: decode(s)
279
280      Return the Python representation of *s* (a :class:`str` or
281      :class:`unicode` instance containing a JSON document)
282
283   .. method:: raw_decode(s)
284
285      Decode a JSON document from *s* (a :class:`str` or :class:`unicode`
286      beginning with a JSON document) and return a 2-tuple of the Python
287      representation and the index in *s* where the document ended.
288
289      This can be used to decode a JSON document from a string that may have
290      extraneous data at the end.
291
292
293.. class:: JSONEncoder([skipkeys[, ensure_ascii[, check_circular[, allow_nan[, sort_keys[, indent[, separators[, encoding[, default]]]]]]]]])
294
295   Extensible JSON encoder for Python data structures.
296
297   Supports the following objects and types by default:
298
299   +-------------------+---------------+
300   | Python            | JSON          |
301   +===================+===============+
302   | dict              | object        |
303   +-------------------+---------------+
304   | list, tuple       | array         |
305   +-------------------+---------------+
306   | str, unicode      | string        |
307   +-------------------+---------------+
308   | int, long, float  | number        |
309   +-------------------+---------------+
310   | True              | true          |
311   +-------------------+---------------+
312   | False             | false         |
313   +-------------------+---------------+
314   | None              | null          |
315   +-------------------+---------------+
316
317   To extend this to recognize other objects, subclass and implement a
318   :meth:`default` method with another method that returns a serializable object
319   for ``o`` if possible, otherwise it should call the superclass implementation
320   (to raise :exc:`TypeError`).
321
322   If *skipkeys* is ``False`` (the default), then it is a :exc:`TypeError` to
323   attempt encoding of keys that are not str, int, long, float or None.  If
324   *skipkeys* is ``True``, such items are simply skipped.
325
326   If *ensure_ascii* is ``True`` (the default), the output is guaranteed to be
327   :class:`str` objects with all incoming unicode characters escaped.  If
328   *ensure_ascii* is ``False``, the output will be a unicode object.
329
330   If *check_circular* is ``True`` (the default), then lists, dicts, and custom
331   encoded objects will be checked for circular references during encoding to
332   prevent an infinite recursion (which would cause an :exc:`OverflowError`).
333   Otherwise, no such check takes place.
334
335   If *allow_nan* is ``True`` (the default), then ``NaN``, ``Infinity``, and
336   ``-Infinity`` will be encoded as such.  This behavior is not JSON
337   specification compliant, but is consistent with most JavaScript based
338   encoders and decoders.  Otherwise, it will be a :exc:`ValueError` to encode
339   such floats.
340
341   If *sort_keys* is ``True`` (the default), then the output of dictionaries
342   will be sorted by key; this is useful for regression tests to ensure that
343   JSON serializations can be compared on a day-to-day basis.
344
345   If *indent* is a non-negative integer (it is ``None`` by default), then JSON
346   array elements and object members will be pretty-printed with that indent
347   level.  An indent level of 0 will only insert newlines.  ``None`` is the most
348   compact representation.
349
350   If specified, *separators* should be an ``(item_separator, key_separator)``
351   tuple.  The default is ``(', ', ': ')``.  To get the most compact JSON
352   representation, you should specify ``(',', ':')`` to eliminate whitespace.
353
354   If specified, *default* is a function that gets called for objects that can't
355   otherwise be serialized.  It should return a JSON encodable version of the
356   object or raise a :exc:`TypeError`.
357
358   If *encoding* is not ``None``, then all input strings will be transformed
359   into unicode using that encoding prior to JSON-encoding.  The default is
360   UTF-8.
361
362
363   .. method:: default(o)
364
365      Implement this method in a subclass such that it returns a serializable
366      object for *o*, or calls the base implementation (to raise a
367      :exc:`TypeError`).
368
369      For example, to support arbitrary iterators, you could implement default
370      like this::
371
372         def default(self, o):
373            try:
374                iterable = iter(o)
375            except TypeError:
376                pass
377            else:
378                return list(iterable)
379            return JSONEncoder.default(self, o)
380
381
382   .. method:: encode(o)
383
384      Return a JSON string representation of a Python data structure, *o*.  For
385      example::
386
387        >>> JSONEncoder().encode({"foo": ["bar", "baz"]})
388        '{"foo": ["bar", "baz"]}'
389
390
391   .. method:: iterencode(o)
392
393      Encode the given object, *o*, and yield each string representation as
394      available.  For example::
395
396            for chunk in JSONEncoder().iterencode(bigobject):
397                mysocket.write(chunk)