19.2. json — JSON 编码器和解码器 ¶

19.2.1. 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 像文件对象 ) using this conversion table .

若 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 .

json module always produces str objects, not bytes objects. Therefore, fp.write() must support str input.

若 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.

若 check_circular is false (default: True ), then the circular reference check for container types will be skipped and a circular reference will result in an OverflowError (or worse).

若 allow_nan is false (default: True ), then it will be a ValueError to serialize out of range float values ( nan , inf , -inf ) in strict compliance of the JSON specification. If allow_nan is true, their JavaScript equivalents ( NaN , Infinity , -Infinity ) will be used.

若 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 "\t" ), that string is used to indent each level.

3.2 版改变： Allow strings for indent in addition to integers.

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.

3.4 版改变： 使用 (',', ': ') as default if indent 不是 None .

If specified, default should be a function that gets called for objects that can’t otherwise be serialized. It should return a JSON encodable version of the object or raise a TypeError . If not specified, TypeError 被引发。

若 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 被使用。

3.6 版改变： All optional parameters are now 仅关键词 .

注意

不像 pickle and marshal , JSON is not a framed protocol, so trying to serialize multiple objects with repeated calls to dump() using the same fp will result in an invalid JSON file.

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 using this conversion table . The arguments have the same meaning as in dump() .

注意

Keys in key/value pairs of JSON are always of the type str . When a dictionary is converted into JSON, all the keys of the dictionary are coerced to strings. As a result of this, if a dictionary is converted into JSON and then back into a dictionary, the dictionary may not equal the original one. That is, loads(dumps(x)) != x if x has non-string keys.

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 文本文件 or 二进制文件 containing a JSON document) to a Python object using this conversion table .

object_hook is an optional function that will be called with the result of any object literal decoded (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.

3.1 版改变： 添加支持 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.

3.1 版改变： 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 会被引发。

3.6 版改变： All optional parameters are now 仅关键词 .

3.6 版改变： fp can now be a 二进制文件 . 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 会被引发。

3.6 版改变： s can now be of type bytes or bytearray . The input encoding should be UTF-8, UTF-16 or UTF-32.

19.2.2. 编码器和解码器 ¶

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 ，和 -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.

3.1 版改变： 添加支持 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.

若 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 会被引发。

3.6 版改变： All parameters are now 仅关键词 .

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

3.4 版改变： 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 ).

若 skipkeys is false (the default), then it is a TypeError to attempt encoding of keys that are not str , int , float or None 。若 skipkeys is true, such items are simply skipped.

若 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.

若 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.

若 allow_nan is true (the default), then NaN , Infinity ，和 -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.

若 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.

若 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 "\t" ), that string is used to indent each level.

3.2 版改变： Allow strings for indent in addition to integers.

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.

3.4 版改变： 使用 (',', ': ') as default if indent 不是 None .

If specified, default should be a function that gets called for objects that can’t otherwise be serialized. It should return a JSON encodable version of the object or raise a TypeError . If not specified, TypeError 被引发。

3.6 版改变： All parameters are now 仅关键词 .

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 。例如：

>>> 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 ¶

行对应 pos .

colno ¶

列对应 pos .

3.5 版新增。

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.

>>> # 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
									

>>> weird_json = '{"x": 1, "x": 2, "x": 3}'
>>> json.loads(weird_json)
{'x': 3}
									

19.2.5. 命令行接口 ¶

源代码： Lib/json/tool.py

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)