# `json` --- JSON 編碼器與解碼器

原始碼： Lib/json/__init__.py

JSON (JavaScript Object Notation) 是一個輕量化的資料交換格式，在 RFC 7159（其廢棄了 RFC 4627）及 ECMA-404 裡面有詳細說明，它啟發自 JavaScript 的物件字面語法 (object literal syntax)（雖然它並不是 JavaScript 的嚴格子集 [1]）。

警告

當剖析無法信任來源的 JSON 資料時要小心。一段惡意的 JSON 字串可能會導致解碼器耗費大量 CPU 與記憶體資源。建議限制剖析資料的大小。

`json` 為標準函式庫 `marshal` 與 `pickle` 模組的使用者提供熟悉的 API。

對基本 Python 物件階層進行編碼：

    >>> 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}]'

美化輸出：

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

JSON 解碼：

    >>> 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 super().default(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)

更詳盡的文件請見 命令列介面。

備註

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.

備註

This module's encoders and decoders preserve input and output order by default. Order is only lost if the underlying containers are unordered.

## 基本用法

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.

  如果 check_circular 設為 false（預設是 `True`），則針對不同容器型別的循環參照 (circular reference) 的檢查將會被跳過，若有循環參照則最後將引發 `RecursionError` （或者更糟的錯誤）。

  如果 allow_nan 為 false（預設值：`True`），則序列化超出嚴格 JSON 規範之範圍的 `float` 值 (`nan`, `inf`, `-inf`) 會引發 `ValueError`。如果 allow_nan 為 true，則將使用它們的 JavaScript 等效項 (`NaN`, `Infinity`, `-Infinity`)。

  如果 indent 是非負整數或字串，則 JSON 陣列元素和物件成員將使用該縮排等級進行漂亮列印。縮排等級 0、負數或 `""` 只會插入換行符號。`None`（預設值）選擇最緊湊的表示法。使用正整數縮排可以在每層縮排數量相同的空格。如果 indent 是一個字串（例如 `"\t"`），則該字串用於縮排每個層級。

  在 3.2 版的變更: 除了整數之外，還允許使用字串進行 indent。

  如果有指定，separators 應該是一個 `(item_separator, key_separator)` 元組。如果 indent 為 `None` 則預設為 `(', ', ': ')`，否則預設為 `(',', ': ')`。要獲得最緊湊的 JSON 表示形式，你應該指定 `(',', ':')` 來消除空格。

  在 3.4 版的變更: 如果 indent 不是 `None`，則用 `(',', ': ')` 當預設值

  如果有指定，default 應該是一個為無法序列化的物件呼叫的函式。它應該傳回物件的 JSON 可編碼版本或引發 `TypeError`。如果未指定，則會引發 `TypeError`。

  如果 sort_keys 為 true（預設值：`False`），則字典的輸出將按鍵排序。

  若要使用自訂 `JSONEncoder` 子類別（例如覆寫 `default()` 方法來序列化其他型別的子類別），請使用 cls kwarg 指定它；否則使用 `JSONEncoder`。

  在 3.6 版的變更: 所有可選參數現在都是僅限關鍵字了。

  備註

  與 `pickle` 和 `marshal` 不同，JSON 不是框架協定，因此嘗試使用相同的 fp 重複呼叫 `dump()` 來序列化多個物件將導致無效的 JSON 檔案。

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)

  使用此轉換表來將 obj 序列化為 JSON 格式化 `str`。這些引數與 `dump()` 中的意義相同。

  備註

  JSON 鍵/值對中的鍵始終為 `str` 型別。當字典轉換為 JSON 時，字典的所有鍵都被強制轉換為字串。因此，如果將字典轉換為 JSON，然後再轉換回字典，則該字典可能不等於原始字典。也就是說，如果 x 有非字串鍵，則 `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)

  使用此轉換表來將 fp（一個支援 `.read()`、包含 JSON 文件的文字檔案或二進位檔案）反序列化為 Python 物件。

  object_hook 是一個可選函式，將使用任何物件文本解碼的結果（一個 `dict`）來呼叫它。將使用 object_hook 的回傳值而不是 `dict`。此功能可用於實作自訂解碼器（例如 JSON-RPC 類別提示）。

  object_pairs_hook 是一個可選函式，將使用使用有序對列表解碼的任何物件文本的結果來呼叫該函式。將使用 object_pairs_hook 的回傳值而不是 `dict`。此功能可用於實作自訂解碼器。如果也定義了 object_hook，則 object_pairs_hook 優先。

  在 3.1 版的變更: 新增對於 object_pairs_hook 的支援。

  如有指定 parse_float，將使用要解碼的每個 JSON 浮點數字串進行呼叫。預設情況下，這相當於 `float(num_str)`。這可用於將另一種資料型別或剖析器用於 JSON 浮點（例如 `decimal.Decimal`）。

  如有指定 parse_int，將使用要解碼的每個 JSON 整數字串進行呼叫。預設情況下，這相當於 `int(num_str)`。這可用於對 JSON 整數使用另一種資料型別或剖析器（例如 `float`）。

  在 3.11 版的變更: `int()` 預設的 parse_int 現在對於整數字串有長度上限，上限是直譯器的整數字串轉換長度限制，這能防止阻斷服務攻擊 (denial of service attacks)。

  如果 parse_constant 有值，那麼以 `'-Infinity'`、`'Infinity'` 或 `'NaN'` 字串其中之一來呼叫。這也可用於在遇到無效的 JSON 數字時引發一個例外。

  在 3.1 版的變更: parse_constant 不再以 'null'、 'true'、 'false' 呼叫了。

  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 版的變更: 所有可選參數現在都是僅限關鍵字了。

  在 3.6 版的變更: fp can now be a binary file. The input encoding should be UTF-8, UTF-16 or UTF-32.

json.loads(s, *, 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()`.

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

  在 3.6 版的變更: s can now be of type `bytes` or `bytearray`. The input encoding should be UTF-8, UTF-16 or UTF-32.

  在 3.9 版的變更: 關鍵字引數 encoding 已經被刪除。

## 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. If object_hook is also defined, the object_pairs_hook takes priority.

  在 3.1 版的變更: 新增對於 object_pairs_hook 的支援。

  如有指定 parse_float，將使用要解碼的每個 JSON 浮點數字串進行呼叫。預設情況下，這相當於 `float(num_str)`。這可用於將另一種資料型別或剖析器用於 JSON 浮點（例如 `decimal.Decimal`）。

  如有指定 parse_int，將使用要解碼的每個 JSON 整數字串進行呼叫。預設情況下，這相當於 `int(num_str)`。這可用於對 JSON 整數使用另一種資料型別或剖析器（例如 `float`）。

  如果 parse_constant 有值，那麼以 `'-Infinity'`、`'Infinity'` 或 `'NaN'` 字串其中之一來呼叫。這也可用於在遇到無效的 JSON 數字時引發一個例外。

  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 版的變更: 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 版的變更: 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), a `TypeError` will be raised when trying to encode 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 a `RecursionError`). 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 是非負整數或字串，則 JSON 陣列元素和物件成員將使用該縮排等級進行漂亮列印。縮排等級 0、負數或 `""` 只會插入換行符號。`None`（預設值）選擇最緊湊的表示法。使用正整數縮排可以在每層縮排數量相同的空格。如果 indent 是一個字串（例如 `"\t"`），則該字串用於縮排每個層級。

  在 3.2 版的變更: 除了整數之外，還允許使用字串進行 indent。

  如果有指定，separators 應該是一個 `(item_separator, key_separator)` 元組。如果 indent 為 `None` 則預設為 `(', ', ': ')`，否則預設為 `(',', ': ')`。要獲得最緊湊的 JSON 表示形式，你應該指定 `(',', ':')` 來消除空格。

  在 3.4 版的變更: 如果 indent 不是 `None`，則用 `(',', ': ')` 當預設值

  如果有指定，default 應該是一個為無法序列化的物件呼叫的函式。它應該傳回物件的 JSON 可編碼版本或引發 `TypeError`。如果未指定，則會引發 `TypeError`。

  在 3.6 版的變更: 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 super().default(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)

## 例外

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.

  在 3.5 版被加入.

## 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.

### 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.

### 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.

### 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}

object_parts_hook 參數可以被使用來改變此行為。

### 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.

### 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`.

## 命令列介面

原始碼： 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 版的變更: 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.

### 命令列選項

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.

  在 3.5 版被加入.

--no-ensure-ascii

  Disable escaping of non-ascii characters, see `json.dumps()` for more information.

  在 3.9 版被加入.

--json-lines

  Parse every input line as separate JSON object.

  在 3.8 版被加入.

--indent, --tab, --no-indent, --compact

  Mutually exclusive options for whitespace control.

  在 3.9 版被加入.

-h, --help

  Show the help message.

註解

[ 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.