# 19.2. `json` — JSON encoder and decoder

Source code: Lib/json/__init__.py

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('\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 `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 -m json.tool
    Expecting property name enclosed in double quotes: line 1 column 2 (char 1)

See Command Line Interface for detailed documentation.

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.

## 19.2.1. 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) using this conversion table.

  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. If allow_nan is true, their JavaScript equivalents (`NaN`, `Infinity`, `-Infinity`) will be used.

  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.

  If specified, separators should be an `(item_separator, key_separator)` tuple. The default is `(', ', ': ')` if indent is `None` and `(',', ': ')` otherwise. To get the most compact JSON representation, you should specify `(',', ':')` to eliminate whitespace.

  Changed in version 3.4: Use `(',', ': ')` as default if indent is not `None`.

  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.

  Changed in version 3.6: All optional parameters are now keyword-only.

  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.

`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` using this conversion table. 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, *, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)

  Deserialize fp (a `.read()`-supporting text file or binary file containing a JSON document) to a Python object using this conversion table.

  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.

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

  Changed in version 3.6: All optional parameters are now keyword-only.

  Changed in version 3.6: fp can now be a binary file. The input encoding should be UTF-8, UTF-16 or UTF-32.

`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`, `bytes` or `bytearray` instance containing a JSON document) to a Python object using this conversion table.

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

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

  Changed in version 3.6: s can now be of type `bytes` or `bytearray`. The input encoding should be UTF-8, UTF-16 or UTF-32.

## 19.2.2. 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'`. 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 `JSONDecodeError` will be raised.

  Changed in version 3.6: All parameters are now keyword-only.

  `decode`(s)

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

    `JSONDecodeError` will be raised if the given JSON document is not valid.

  `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, int- & float-derived Enums | number
  True | true
  False | false
  None | null

  Changed in version 3.4: Added support for int- and float-derived Enum classes.

  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.

  If specified, separators should be an `(item_separator, key_separator)` tuple. The default is `(', ', ': ')` if indent is `None` and `(',', ': ')` otherwise. To get the most compact JSON representation, you should specify `(',', ':')` to eliminate whitespace.

  Changed in version 3.4: Use `(',', ': ')` as default if indent is not `None`.

  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.

  Changed in version 3.6: All parameters are now keyword-only.

  `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)

## 19.2.3. Exceptions

exception `json.``JSONDecodeError`(msg, doc, pos)

  Subclass of `ValueError` with the following additional attributes:

  `msg`

    The unformatted error message.

  `doc`

    The JSON document being parsed.

  `pos`

    The start index of doc where parsing failed.

  `lineno`

    The line corresponding to pos.

  `colno`

    The column corresponding to pos.

  New in version 3.5.

## 19.2.4. 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.

### 19.2.4.1. 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.

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 directly address the issue of character encodings.

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.

### 19.2.4.2. 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.

### 19.2.4.3. 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)
    {'x': 3}

The object_pairs_hook parameter can be used to alter this behavior.

### 19.2.4.4. 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.

### 19.2.4.5. 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`.

## 19.2.5. Command Line Interface

Source code: Lib/json/tool.py

The `json.tool` module provides a simple command line interface to validate and pretty-print JSON objects.

If the optional `infile` and `outfile` arguments are not specified, `sys.stdin` and `sys.stdout` will be used respectively:

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

Changed in version 3.5: The output is now in the same order as the input. Use the `--sort-keys` option to sort the output of dictionaries alphabetically by key.

### 19.2.5.1. Command line options

`infile```

  The JSON file to be validated or pretty-printed:

      $ python -m json.tool mp_films.json
      [
          {
              "title": "And Now for Something Completely Different",
              "year": 1971
          },
          {
              "title": "Monty Python and the Holy Grail",
              "year": 1975
          }
      ]

  If infile is not specified, read from `sys.stdin`.

`outfile```

  Write the output of the infile to the given outfile. Otherwise, write it to `sys.stdout`.

`--sort-keys```

  Sort the output of dictionaries alphabetically by key.

  New in version 3.5.

`-h````,` `--help```

  Show the help message.

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.