简介
JSON(JavaScript Object Notation)是一种轻量级的数据交换格式。它是基于 ECMAScript(欧洲计算机协会制定的 js 规范)的一个子集，采用完全独立于编程语言的文本格式来存储和表示数据。json 主要的优点有：
- 层次结构简洁清晰
- 易于阅读和编写
- 易于机器解析和生成
- 网络传输效率高
JSON 数据格式
- 对象：以‘{’开始，以‘}’结束，和 python 中的字典类似
- 数组：以‘[’开始，以‘]’结束，和 python 中的列表类似
- 整型、浮点型、布尔类型和 NULL
- 字符串类型：以‘“’开始，以‘”’结束
多个数据之间使用逗号分隔。
从 python 到 JSON
import json
students = [
{'name':'zhangsan','num':'100'},
{'name':'lisi','num':'101'}
]
print(students)
stu_json = json.dumps(students)
print(type(stu_json))
print(stu_json)
结果为：
[{'name': 'zhangsan', 'num': '100'}, {'name': 'lisi', 'num': '101'}]
<class 'str'>
[{"name": "zhangsan", "num": "100"}, {"name": "lisi", "num": "101"}]
使用 dumps 进行的数据转换返回的结果为 str 类型，同时可以看到 python 中的单引号被转换为了双引号。
dumps
def 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``.
If ``skipkeys`` is true then ``dict`` keys that are not basic types
(``str``, ``int``, ``float``, ``bool``, ``None``) will be skipped
instead of raising a ``TypeError``.
If ``ensure_ascii`` is false, then the return value can contain non-ASCII
characters if they appear in strings contained in ``obj``. Otherwise, all
such characters are escaped in JSON strings.
If ``check_circular`` is false, 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, then it will be a ``ValueError`` to
serialize out of range ``float`` values (``nan``, ``inf``, ``-inf``) in
strict compliance of the JSON specification, instead of using the
JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``).
If ``indent`` is a non-negative integer, then JSON array elements and
object members will be pretty-printed with that indent level. An indent
level of 0 will only insert newlines. ``None`` is the most compact
representation.
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.
``default(obj)`` is a function that should return a serializable version
of obj or raise TypeError. The default simply raises TypeError.
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.
"""
# cached encoder
if (not skipkeys and ensure_ascii and
check_circular and allow_nan and
cls is None and indent is None and separators is None and
default is None and not sort_keys and not kw):
return _default_encoder.encode(obj)
if cls is None:
cls = JSONEncoder
return cls(
skipkeys=skipkeys, ensure_ascii=ensure_ascii,
check_circular=check_circular, allow_nan=allow_nan, indent=indent,
separators=separators, default=default, sort_keys=sort_keys,
**kw).encode(obj)
将 json 数据写入文件
使用 dumps 可以将原始数据转换为 json 格式的 str，而使用 dumps 则可以将转换后的 json 数据写入到文件中。
import json
students = [
{'name':'zhangsan','num':'100'},
{'name':'lisi','num':'101'}
]
with open('js.json','w') as fp:
json.dump(students,fp)
fp.close()
这样就将转换后的 json 数据写入了 js.json 文件。
dump
def 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).
If ``skipkeys`` is true then ``dict`` keys that are not basic types
(``str``, ``int``, ``float``, ``bool``, ``None``) will be skipped
instead of raising a ``TypeError``.
If ``ensure_ascii`` is false, then the strings written to ``fp`` can
contain non-ASCII characters if they appear in strings contained in
``obj``. Otherwise, all such characters are escaped in JSON strings.
If ``check_circular`` is false, 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, then it will be a ``ValueError`` to
serialize out of range ``float`` values (``nan``, ``inf``, ``-inf``)
in strict compliance of the JSON specification, instead of using the
JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``).
If ``indent`` is a non-negative integer, then JSON array elements and
object members will be pretty-printed with that indent level. An indent
level of 0 will only insert newlines. ``None`` is the most compact
representation.
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.
``default(obj)`` is a function that should return a serializable version
of obj or raise TypeError. The default simply raises TypeError.
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.
"""
# cached encoder
if (not skipkeys and ensure_ascii and
check_circular and allow_nan and
cls is None and indent is None and separators is None and
default is None and not sort_keys and not kw):
iterable = _default_encoder.iterencode(obj)
else:
if cls is None:
cls = JSONEncoder
iterable = cls(skipkeys=skipkeys, ensure_ascii=ensure_ascii,
check_circular=check_circular, allow_nan=allow_nan, indent=indent,
separators=separators,
default=default, sort_keys=sort_keys, **kw).iterencode(obj)
# could accelerate with writelines in some versions of Python, at
# a debuggability cost
for chunk in iterable:
fp.write(chunk)
从 JSON 到 python
将 json 字符串转换为 python 对象
import json
json_data = """
[
{"name": "zhangsan", "num": "100"},
{"name": "lisi", "num": "101"}
]
"""
data = json.loads(json_data,encoding='utf-8')
print(type(data))
print(data)
结果为：
<class 'list'>
[{'name': 'zhangsan', 'num': '100'}, {'name': 'lisi', 'num': '101'}]
可知，上边的代码段使用 loads 将字符串转换为了 python 中的 list。可以看出 JSON 的实质是字符串。
loads
def 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.
``object_hook`` is an optional function that will be called with the
result of any object literal decode (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.
``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.
To use a custom ``JSONDecoder`` subclass, specify it with the ``cls``
kwarg; otherwise ``JSONDecoder`` is used.
The ``encoding`` argument is ignored and deprecated.
"""
if isinstance(s, str):
if s.startswith('\ufeff'):
raise JSONDecodeError("Unexpected UTF-8 BOM (decode using utf-8-sig)",
s, 0)
else:
if not isinstance(s, (bytes, bytearray)):
raise TypeError('the JSON object must be str, bytes or bytearray, '
'not {!r}'.format(s.__class__.__name__))
s = s.decode(detect_encoding(s), 'surrogatepass')
if (cls is None and object_hook is None and
parse_int is None and parse_float is None and
parse_constant is None and object_pairs_hook is None and not kw):
return _default_decoder.decode(s)
if cls is None:
cls = JSONDecoder
if object_hook is not None:
kw['object_hook'] = object_hook
if object_pairs_hook is not None:
kw['object_pairs_hook'] = object_pairs_hook
if parse_float is not None:
kw['parse_float'] = parse_float
if parse_int is not None:
kw['parse_int'] = parse_int
if parse_constant is not None:
kw['parse_constant'] = parse_constant
return cls(**kw).decode(s)
从文件中读取 json
之前使用 loads 将 json 字符串转换为了 python 对象，而同样使用 load 则可以从文件中读取 json。
import json
with open('js.json','r',encoding='utf-8') as fp:
json_data = json.load(fp)
print(type(json_data))
print(json_data)
fp.close()
结果为：
<class 'list'>
[{'name': 'zhangsan', 'num': '100'}, {'name': 'lisi', 'num': '101'}]
load
def 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.
``object_hook`` is an optional function that will be called with the
result of any object literal decode (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.
To use a custom ``JSONDecoder`` subclass, specify it with the ``cls``
kwarg; otherwise ``JSONDecoder`` is used.
"""
return loads(fp.read(),
cls=cls, object_hook=object_hook,
parse_float=parse_float, parse_int=parse_int,
parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)