18.2. "json" — JSON encoder and decoder
***************************************

New in version 2.6.

JSON (JavaScript Object Notation), specified by **RFC 7159** (which
obsoletes **RFC 4627**) and by ECMA-404, is a lightweight data
interchange format inspired by JavaScript object literal syntax
(although it is not a strict subset of JavaScript [1] ).

"json" exposes an API familiar to users of the standard library
"marshal" and "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, separators=(',', ': '))
   {
       "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 "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', ']']

Using "json.tool" from the shell to validate and pretty-print:

   $ echo '{"json":"obj"}' | python -m json.tool
   {
       "json": "obj"
   }
   $ echo '{1.2:3.4}' | python -mjson.tool
   Expecting property name enclosed in double quotes: line 1 column 2 (char 1)

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.


18.2.1. Basic Usage
===================

json.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 *file-like object*) using this conversion
   table.

   If *skipkeys* is true (default: "False"), then dict keys that are
   not of a basic type ("str", "unicode", "int", "long", "float",
   "bool", "None") will be skipped instead of raising a "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 "str" instance consisting of ASCII characters only.  If
   *ensure_ascii* is false, some chunks written to *fp* may be
   "unicode" instances.  This usually happens because the input
   contains unicode strings or the *encoding* parameter is used.
   Unless "fp.write()" explicitly understands "unicode" (as in
   "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 "OverflowError" (or worse).

   If *allow_nan* is false (default: "True"), then it will be a
   "ValueError" to serialize out of range "float" values ("nan",
   "inf", "-inf") in strict compliance of the JSON specification. If
   *allow_nan* is true, their JavaScript equivalents ("NaN",
   "Infinity", "-Infinity") will be used.

   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.

   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.  By default, "(', ', ': ')" are used.  To
   get the most compact JSON representation, you should specify "(',',
   ':')" to eliminate whitespace.

   *encoding* is the character encoding for str instances, default is
   UTF-8.

   If specified, *default* should be 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 "TypeError".  If
   not specified, "TypeError" is raised.

   If *sort_keys* is true (default: "False"), then the output of
   dictionaries will be sorted by key.

   To use a custom "JSONEncoder" subclass (e.g. one that overrides the
   "default()" method to serialize additional types), specify it with
   the *cls* kwarg; otherwise "JSONEncoder" is used.

   Note: Unlike "pickle" and "marshal", JSON is not a framed
     protocol so trying to serialize more objects with repeated calls
     to "dump()" and the same *fp* will result in an invalid JSON
     file.

json.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 "str" using this conversion
   table.  If *ensure_ascii* is false, the result may contain non-
   ASCII characters and the return value may be a "unicode" instance.

   The arguments have the same meaning as in "dump()".

   Note: Keys in key/value pairs of JSON are always of the type
     "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.

json.load(fp[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[, **kw]]]]]]]])

   Deserialize *fp* (a ".read()"-supporting *file-like object*
   containing a JSON document) to a Python object using this
   conversion table.

   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 "unicode"
   object and passed to "loads()".

   *object_hook* is an optional function that will be called with the
   result of any object literal decoded (a "dict").  The return value
   of *object_hook* will be used instead of the "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 "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, "collections.OrderedDict()" will remember
   the order of insertion). If *object_hook* is also defined, the
   *object_pairs_hook* takes priority.

   Changed in version 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. "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. "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.

   Changed in version 2.7: *parse_constant* doesn’t get called on
   ‘null’, ‘true’, ‘false’ anymore.

   To use a custom "JSONDecoder" subclass, specify it with the "cls"
   kwarg; otherwise "JSONDecoder" is used.  Additional keyword
   arguments will be passed to the constructor of the class.

json.loads(s[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[, **kw]]]]]]]])

   Deserialize *s* (a "str" or "unicode" instance containing a JSON
   document) to a Python object using this conversion table.

   If *s* is a "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
   "unicode" first.

   The other arguments have the same meaning as in "load()".


18.2.2. Encoders and Decoders
=============================

class json.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 "str"
   objects decoded by this instance (UTF-8 by default).  It has no
   effect when decoding "unicode" objects.

   Note that currently only encodings that are a superset of ASCII
   work, strings of other encodings should be passed in as "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 "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
   "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, "collections.OrderedDict()" will remember the order of
   insertion). If *object_hook* is also defined, the
   *object_pairs_hook* takes priority.

   Changed in version 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. "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. "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.

   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'".

   If the data being deserialized is not a valid JSON document, a
   "ValueError" will be raised.

   decode(s)

      Return the Python representation of *s* (a "str" or "unicode"
      instance containing a JSON document).

   raw_decode(s)

      Decode a JSON document from *s* (a "str" or "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 json.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
   "default()" method with another method that returns a serializable
   object for "o" if possible, otherwise it should call the superclass
   implementation (to raise "TypeError").

   If *skipkeys* is false (the default), then it is a "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 "str" instances consisting of ASCII characters only. If
   *ensure_ascii* is false, a result may be a "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 "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 "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.

   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.  By default, "(', ', ': ')" are used.  To
   get the most compact JSON representation, you should specify "(',',
   ':')" to eliminate whitespace.

   If specified, *default* should be 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 "TypeError".  If
   not specified, "TypeError" is raised.

   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.

   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 "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 JSONEncoder.default(self, o)

   encode(o)

      Return a JSON string representation of a Python data structure,
      *o*.  For example:

         >>> JSONEncoder().encode({"foo": ["bar", "baz"]})
         '{"foo": ["bar", "baz"]}'

   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)


18.2.3. Standard Compliance and Interoperability
================================================

The JSON format is specified by **RFC 7159** and by ECMA-404. This
section details this module’s level of compliance with the RFC. For
simplicity, "JSONEncoder" and "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:

* 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.


18.2.3.1. Character Encodings
-----------------------------

The RFC requires that JSON be represented using either UTF-8, UTF-16,
or UTF-32, with UTF-8 being the recommended default for maximum
interoperability. 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.

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.

The RFC prohibits adding a byte order mark (BOM) to the start of a
JSON text, and this module’s serializer does not add a BOM to its
output. The RFC permits, but does not require, JSON deserializers to
ignore an initial BOM in their input.  This module’s deserializer
raises a "ValueError" when an initial BOM is present.

The RFC does not explicitly forbid JSON strings which contain byte
sequences that don’t correspond to valid Unicode characters (e.g.
unpaired UTF-16 surrogates), but it does note that they may cause
interoperability problems. By default, this module accepts and outputs
(when present in the original "str") code points for such sequences.


18.2.3.2. 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.


18.2.3.3. Repeated Names Within an Object
-----------------------------------------

The RFC specifies that the names within a JSON object should be
unique, but does not mandate 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.


18.2.3.4. Top-level Non-Object, Non-Array Values
------------------------------------------------

The old version of JSON specified by the obsolete **RFC 4627**
required that the top-level value of a JSON text must be either a JSON
object or array (Python "dict" or "list"), and could not be a JSON
null, boolean, number, or string value.  **RFC 7159** removed that
restriction, and this module does not and has never implemented that
restriction in either its serializer or its deserializer.

Regardless, for maximum interoperability, you may wish to voluntarily
adhere to the restriction yourself.


18.2.3.5. Implementation Limitations
------------------------------------

Some JSON deserializer implementations may set limits on:

* the size of accepted JSON texts

* the maximum level of nesting of JSON objects and arrays

* the range and precision of JSON numbers

* the content and maximum length of JSON strings

This module does not impose any such limits beyond those of the
relevant Python datatypes themselves or the Python interpreter itself.

When serializing to JSON, beware any such limitations in applications
that may consume your JSON.  In particular, it is common for JSON
numbers to be deserialized into IEEE 754 double precision numbers and
thus subject to that representation’s range and precision limitations.
This is especially relevant when serializing Python "int" values of
extremely large magnitude, or when serializing instances of “exotic”
numerical types such as "decimal.Decimal".

-[ Footnotes ]-

[1] As noted in the errata for RFC 7159, JSON permits literal
    U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR)
    characters in strings, whereas JavaScript (as of ECMAScript
    Edition 5.1) does not.
