JSON模块

JSON (JavaScript Object Notation) 是一种轻量级的数据交换格式。

JSON模块属于Python的标准库，在安装python时会被默认安装，使用时直接import即可，无需单独安装。

Python可以使用 json 模块来对 JSON 数据进行编解码。

先看看帮助文档：

>>> import json
>>> dir(json)
['JSONDecodeError', 'JSONDecoder', 'JSONEncoder', '__all__', '__author__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__path__', '__spec__', '__version__', '_default_decoder', '_default_encoder', 'codecs', 'decoder', 'detect_encoding', 'dump', 'dumps', 'encoder', 'load', 'loads', 'scanner']

JSON模块包括两个常用的内置函数：

• json.dumps()：对数据进行编码，序列化。将python对象编码为json字符串。
• json.loads()：对数据进行解码，反序列化。将json字符串解码为python对象。

在json的编解码过程中，python的原始类型与json类型会相互转换。

Python 编码为 JSON 类型转换对应表：

JSON 解码为 Python 类型转换对应表：

用法如下，文档太长参数说明太多，这里只贴了语法格式部分：

>>> help(json.dumps)
Help on function dumps in module 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``.

>>> help(json.loads)
Help on function loads in module 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.

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

注意上述文档说的是str，也就是这两个方法操作的是JSON格式的字符串。

示例：

>>> users=[{'name':'keith','age':18,'tel':110},{'name':'jerry','age':36,'tel':112}]
>>> import json

>>> j=json.dumps(users)
>>> j
'[{"name": "keith", "age": 18, "tel": 110}, {"name": "jerry", "age": 36, "tel": 112}]'

>>> users2=json.loads(users_json)
>>> users2
[{'name': 'keith', 'age': 18, 'tel': 110}, {'name': 'jerry', 'age': 36, 'tel': 112}]

默认的格式并不美观，常见的JSON文档都是带缩进的，这里当然也可以，json.dumps()提供了可选参数。

>>> print(json.dumps(users,sort_keys=True, indent=4, separators=(',', ': ')))
[
    {
        "age": 18,
        "name": "keith",
        "tel": 110
    },
    {
        "age": 36,
        "name": "jerry",
        "tel": 112
    }
]

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 "	"), that string is used to indent each level.
缩进是一个非负整数或字符串，默认为None。

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.
分隔符是一个元祖， 包含两个元素，一个是item分隔符，一个是kv分隔符。默认为 (', ', ': ')。

If sort_keys is true (default: False), then the output of dictionaries will be sorted by key.
sort_keys如果为true，则输出的字典会以key排序。默认为False。

---

如果你要处理的是文件而不是字符串，可以使用json.dump()和json.load()来编码和解码JSON数据。

>>> help(json.dump)
Help on function dump in module 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).

>>> help(json.load)
Help on function load in module 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.

示例：

写入JSON数据
with open('data.json', 'w') as f:
    json.dump(data, f)

读取数据
with open('data.json', 'r') as f:
    data = json.load(f)

>>> with open('data.json', 'w') as f:
... 	 json.dump([{'name':'keith','age':18,'tel':110},{'name':'jerry','age':36,'tel':112}], f)
... 
# cat data.json
[{"name": "keith", "age": 18, "tel": 110}, {"name": "jerry", "age": 36, "tel": 112}]

>>> with open('data.json', 'r') as f:
... 	data = json.load(f)
...
>>> data
[{'name': 'keith', 'age': 18, 'tel': 110}, {'name': 'jerry', 'age': 36, 'tel': 112}]

参考：
https://docs.python.org/3/library/json.html
http://www.runoob.com/python/python-json.html
http://www.runoob.com/python3/python3-json.html