"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.
