# 19.2. `json` — JSON 编码和解码器

源代码: Lib/json/__init__.py

JSON (JavaScript Object Notation)，由 RFC 7159 (which obsoletes RFC 4627) 和 ECMA-404 指定，是一个受 JavaScript 的对象字面量语法启发的轻量级数据交换格式，尽管它不仅仅是一个严格意义上的 JavaScript 的字集 1。

`json` 提供了与标准库 `marshal` 和 `pickle` 相似的API接口。

对基本的 Python 对象层次结构进行编码：

    >>> 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"]'

紧凑编码:

    >>> import json
    >>> json.dumps([1, 2, 3, {'4': 5, '6': 7}], separators=(',', ':'))
    '[1,2,3,{"4":5,"6":7}]'

美化输出:

    >>> import json
    >>> print(json.dumps({'4': 5, '6': 7}, sort_keys=True, indent=4))
    {
        "4": 5,
        "6": 7
    }

JSON解码:

    >>> 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']

特殊JSON对象解码:

    >>> 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')

扩展 `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', ']']

从命令行使用 `json.tool` 来验证并美化输出：

    $ 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)

详细文档请参见 Command Line Interface。

注解

JSON 是 YAML 1.2 的一个子集。由该模块的默认设置生成的 JSON （尤其是默认的 “分隔符” 设置值）也是 YAML 1.0 and 1.1 的一个子集。因此该模块也能够用于序列化为 YAML。

## 19.2.1. 基本使用

`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)

  使用这个 conversion table 来序列化 obj 为一个 JSON 格式的流并输出到 fp （一个支持 `.write()` 的 file-like object）。

  如果 skipkeys 是 true （默认为 `False`），那么那些不是基本对象（包括 `str`, `int`、`float`、`bool`、`None`）的字典的键会被跳过；否则引发一个 `TypeError`。

  `json` 模块始终产生 `str` 对象而非 `bytes` 对象。因此，`fp.write()` 必须支持 `str` 输入。

  如果 ensure_ascii 是 true （即默认值），输出保证将所有输入的非 ASCII 字符转义。如果 ensure_ascii 是 false，这些字符会原样输出。

  如果 check_circular 是为假值 (默认为 `True`)，那么容器类型的循环引用检验会被跳过并且循环引用会引发一个 `OverflowError` (或者更糟的情况)。

  如果 allow_nan 是 false（默认为 `True`），那么在对严格 JSON 规格范围外的 `float` 类型值（`nan`、`inf` 和 `-inf`）进行序列化时会引发一个 `ValueError`。如果 allow_nan 是 true，则使用它们的 JavaScript 等价形式（`NaN`、`Infinity` 和 `-Infinity`）。

  如果 indent 是一个非负整数或者字符串，那么 JSON 数组元素和对象成员会被美化输出为该值指定的缩进等级。如果缩进等级为零、负数或者 `""`，则只会添加换行符。`None``（默认值）选择最紧凑的表达。使用一个正整数会让每一层缩进同样数量的空格。如果 *indent* 是一个字符串（比如 ``"\t"`），那个字符串会被用于缩进每一层。

  在 3.2 版更改: 允许使用字符串作为 indent 而不再仅仅是整数。

  当指定时，separators 应当是一个 `(item_separator, key_separator)` 元组。当 indent 为 `None` 时，默认值取 `(', ', ': ')`，否则取 `(',', ': ')`。为了得到最紧凑的 JSON 表达式，你应该指定其为 `(',', ':')` 以消除空白字符。

  在 3.4 版更改: 现当 indent 不是 `None` 时，采用 `(',', ': ')` 作为默认值。

  当 default 被指定时，其应该是一个函数，每当某个对象无法被序列化时它会被调用。它应该返回该对象的一个可以被 JSON 编码的版本或者引发一个 `TypeError`。如果没有被指定，则会直接引发 `TypeError`。

  如果 sort_keys 是 true（默认为 `False`），那么字典的输出会以键的顺序排序。

  为了使用一个自定义的 `JSONEncoder` 子类（比如：覆盖了 `default()` 方法来序列化额外的类型）， 通过 cls 关键字参数来指定；否则将使用 `JSONEncoder`。

  在 3.6 版更改: 所有的可选参数现在是 keyword-only 的了。

  注解

  与 `pickle` 和 `marshal` 不同，JSON 不是一个具有框架的协议，所以尝试多次使用同一个 fp 调用 `dump()` 来序列化多个对象会产生一个不合规的 JSON 文件。

`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)

  使用这个 转换表 将 obj 序列化为 JSON 格式的 `str`。 其参数的含义与 `dump()` 中的相同。

  注解

  JSON 中的键-值对中的键永远是 `str` 类型的。当一个对象被转化为 JSON 时，字典中所有的键都会被强制转换为字符串。这所造成的结果是字典被转换为 JSON 然后转换回字典时可能和原来的不相等。换句话说，如果 x 具有非字符串的键，则有 `loads(dumps(x)) != x`。

`json.``load`(fp, *, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, **kw)

  使用这个 转换表 将 fp (一个支持 `.read()` 并包含一个 JSON 文档的 text file 或者 binary file) 反序列化为一个 Python 对象。

  object_hook 是一个可选的函数，它会被调用于每一个解码出的对象字面量（即一个 `dict`）。object_hook 的返回值会取代原本的 `dict`。这一特性能够被用于实现自定义解码器（如 JSON-RPC 的类型提示)。

  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.

  在 3.1 版更改: 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.

  在 3.1 版更改: 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.

  在 3.6 版更改: 所有的可选参数现在是 keyword-only 的了。

  在 3.6 版更改: 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.

  在 3.6 版更改: 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
  整数 | int
  非整数 | 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.

  在 3.1 版更改: 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.

  在 3.6 版更改: 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

  在 3.4 版更改: 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.

  如果 ensure_ascii 是 true （即默认值），输出保证将所有输入的非 ASCII 字符转义。如果 ensure_ascii 是 false，这些字符会原样输出。

  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.

  如果 indent 是一个非负整数或者字符串，那么 JSON 数组元素和对象成员会被美化输出为该值指定的缩进等级。如果缩进等级为零、负数或者 `""`，则只会添加换行符。`None``（默认值）选择最紧凑的表达。使用一个正整数会让每一层缩进同样数量的空格。如果 *indent* 是一个字符串（比如 ``"\t"`），那个字符串会被用于缩进每一层。

  在 3.2 版更改: 允许使用字符串作为 indent 而不再仅仅是整数。

  当指定时，separators 应当是一个 `(item_separator, key_separator)` 元组。当 indent 为 `None` 时，默认值取 `(', ', ': ')`，否则取 `(',', ': ')`。为了得到最紧凑的 JSON 表达式，你应该指定其为 `(',', ':')` 以消除空白字符。

  在 3.4 版更改: 现当 indent 不是 `None` 时，采用 `(',', ': ')` 作为默认值。

  当 default 被指定时，其应该是一个函数，每当某个对象无法被序列化时它会被调用。它应该返回该对象的一个可以被 JSON 编码的版本或者引发一个 `TypeError`。如果没有被指定，则会直接引发 `TypeError`。

  在 3.6 版更改: 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. 异常

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

  Subclass of `ValueError` with the following additional attributes:

  `msg`

    未格式化的错误消息。

  `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.

  3.5 新版功能.

## 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)

在 3.5 版更改: 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.

  3.5 新版功能.

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

  Show the help message.

脚注

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.