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# 18.2. `json` — JSON encoder and decoder

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).

`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('\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 `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 -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)

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, default=None, sort_keys=False, **kw)

  Serialize obj as a JSON formatted stream to fp (a `.write()`-supporting file-like object).

  If skipkeys is `True` (default: `False`), then dict keys that are not of a basic type (`str`, `int`, `float`, `bool`, `None`) will be skipped instead of raising a `TypeError`.

  The `json` module always produces `str` objects, not `bytes` objects. Therefore, `fp.write()` must support `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 `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, 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.

  Changed in version 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 `TypeError`. The default simply raises `TypeError`.

  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.

`json.``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 `str`. The arguments have the same meaning as in `dump()`.

  Note

  Unlike `pickle` and `marshal`, JSON is not a framed protocol, so trying to serialize multiple objects with repeated calls to `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 `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, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)

  Deserialize fp (a `.read()`-supporting 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 `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 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. `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 3.1: 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=None, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)

  Deserialize s (a `str` instance containing a JSON document) to a Python object.

  The other arguments have the same meaning as in `load()`, except encoding which is ignored and deprecated.

## 18.2.2. Encoders and Decoders

class `json.``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 `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 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. `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'`, `'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'`.

  `decode`(s)

    Return the Python representation of s (a `str` instance containing a JSON document)

  `raw_decode`(s)

    Decode a JSON document from s (a `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 `json.``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 `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, 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 `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 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.

  Changed in version 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 `TypeError`.

  `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 json.JSONEncoder.default(self, o)

  `encode`(o)

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

        >>> json.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 json.JSONEncoder().iterencode(bigobject):
            mysocket.write(chunk)

## 18.2.3. Standard Compliance

The JSON format is specified by RFC 4627. 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:

- 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.

### 18.2.3.1. 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 `Unicode strings`, and thus does not otherwise address the issue of character encodings.

### 18.2.3.2. 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 `dict` or `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 `None`, `bool`, numeric, and `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.

### 18.2.3.3. 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.

### 18.2.3.4. 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.

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