18.2. json
— JSON encoder and decoder¶
JSON (JavaScript Object Notation) is a subset of JavaScript syntax (ECMA-262 3rd edition) used as a lightweight data interchange format.
json
exposes an API familiar to users of the standard library
marshal
and pickle
modules.
Encoding basic Python object hierarchies:
>>> 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}]'
Pretty printing:
>>> import json
>>> print(json.dumps({'4': 5, '6': 7}, sort_keys=True, indent=4))
{
"4": 5,
"6": 7
}
Decoding JSON:
>>> 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]
... return json.JSONEncoder.default(self, 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 -mjson.tool
{
"json": "obj"
}
$ echo '{ 1.2:3.4}' | python -mjson.tool
Expecting property name: line 1 column 2 (char 2)
Note
The JSON produced by this module’s default settings is a subset of YAML, so it may be used as a serializer for that as well.
18.2.1. Basic Usage¶
-
json.
dump
(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, **kw)¶ Serialize obj as a JSON formatted stream to fp (a
.write()
-supporting file-like object).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 aTypeError
.The
json
module always producesstr
objects, notbytes
objects. Therefore,fp.write()
must supportstr
input.If check_circular is
False
(default:True
), then the circular reference check for container types will be skipped and a circular reference will result in anOverflowError
(or worse).If allow_nan is
False
(default:True
), then it will be aValueError
to serialize out of rangefloat
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 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 at ‘t’), that string is used to indent each level.If separators is an
(item_separator, dict_separator)
tuple, then it will be used instead of the default(', ', ': ')
separators.(',', ':')
is the most compact JSON representation.default(obj) is a function that should return a serializable version of obj or raise
TypeError
. The default simply raisesTypeError
.To use a custom
JSONEncoder
subclass (e.g. one that overrides thedefault()
method to serialize additional types), specify it with the cls kwarg; otherwiseJSONEncoder
is used.
-
json.
dumps
(obj, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, **kw)¶ Serialize obj to a JSON formatted
str
. The arguments have the same meaning as indump()
.
-
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.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 thedict
. 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.Changed in version 3.1:
Changed in version 3.1: Added support for 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'
, 'null'
, 'true'
,
'false'
. 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. Additional keyword arguments
will be passed to the constructor of the class.
-
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
instance containing a JSON document) to a Python object.The other arguments have the same meaning as in
load()
, except encoding which is ignored and deprecated.
18.2.2. 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 correspondingfloat
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.Changed in version 3.1:
Changed in version 3.1: Added support for 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'
, 'null'
, 'true'
,
'false'
. This can be used to raise an exception if invalid JSON numbers
are encountered.
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'
.
-
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 number True true False false None null To extend this to recognize other objects, subclass and implement a
default()
method with another method that returns a serializable object foro
if possible, otherwise it should call the superclass implementation (to raiseTypeError
).If skipkeys is
False
(the default), then it is aTypeError
to attempt encoding of keys that are not str, int, float or None. If skipkeys isTrue
, 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 isFalse
, 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 anOverflowError
). Otherwise, no such check takes place.If allow_nan is
True
(the default), thenNaN
,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 aValueError
to encode such floats.If sort_keys is
True
(defaultFalse
), 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.If indent is a non-negative integer (it is
None
by default), 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(', ', ': ')
. To get the most compact JSON representation, you should specify(',', ':')
to eliminate whitespace.If specified, default is 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
.-
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) return json.JSONEncoder.default(self, 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)
-