json — JSON encoder and decoder¶Source code: Lib/json/__init__.py
JSON (JavaScript Object Notation), specified by RFC 7159 (which obsoletes RFC 4627) and by ECMA-404, is a lightweight data interchange format inspired by JavaScript object literal syntax (although it is not a strict subset of JavaScript [1] ).
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]
... # Let the base class default method raise the TypeError
... 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 -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)
See Command Line Interface for detailed documentation.
Note
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.
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.
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 an OverflowError (or worse).
If 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.
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 "\t"),
that string is used to indent each level.
Changed in version 3.2: 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.
Changed in version 3.4: Use (',', ': ') as default if indent is not 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
is raised.
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.
Changed in version 3.6: All optional parameters are now keyword-only.
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().
Note
Unlike 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.
Note
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 file-like object
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.
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'.
This can be used to raise an exception if invalid JSON numbers
are encountered.
Changed in version 3.1: 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 will be raised.
Changed in version 3.6: All optional parameters are now keyword-only.
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 will be raised.
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 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: 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'.
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'.
If the data being deserialized is not a valid JSON document, a
JSONDecodeError will be raised.
Changed in version 3.6: 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.
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 |
Changed in version 3.4: 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), then it is a TypeError to
attempt encoding of 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 an OverflowError).
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.
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 "\t"),
that string is used to indent each level.
Changed in version 3.2: 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.
Changed in version 3.4: Use (',', ': ') as default if indent is not 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
is raised.
Changed in version 3.6: 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 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)
json.JSONDecodeError(msg, doc, pos, end=None)¶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.
New in version 3.5.
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:
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.
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.
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.
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}
The object_pairs_hook parameter can be used to alter this behavior.
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.
Some JSON deserializer implementations may set limits on:
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.
Source code: 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)
Changed in version 3.5: 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.
infileThe 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.
outfileWrite the output of the infile to the given outfile. Otherwise, write it
to sys.stdout.
--sort-keysSort the output of dictionaries alphabetically by key.
New in version 3.5.
-h, --helpShow the help message.
Footnotes
| [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. |