json — JSON encoder and decoder (2024)

Source code: Lib/json/__init__.py

JSON (JavaScript Object Notation), specified byRFC 7159 (which obsoletes RFC 4627) and byECMA-404,is a lightweight data interchange format inspired byJavaScript object literal syntax(although it is not a strict subset of JavaScript [1] ).

Warning

Be cautious when parsing JSON data from untrusted sources. A maliciousJSON string may cause the decoder to consume considerable CPU and memoryresources. Limiting the size of data to be parsed is recommended.

json exposes an API familiar to users of the standard librarymarshal 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 super().default(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.toolExpecting 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 bythis module’s default settings (in particular, the default separatorsvalue) is also a subset of YAML 1.0 and 1.1. This module can thus also beused as a YAML serializer.

Note

This module’s encoders and decoders preserve input and output order bydefault. Order is only lost if the underlying containers are unordered.

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, sort_keys=False, **kw)¶

Serialize obj as a JSON formatted stream to fp (a .write()-supportingfile-like object) using this conversion table.

If skipkeys is true (default: False), then dict keys that are notof a basic type (str, int, float, bool,None) will be skipped instead of raising a TypeError.

The json module always produces str objects, notbytes objects. Therefore, fp.write() must support strinput.

If ensure_ascii is true (the default), the output is guaranteed tohave all incoming non-ASCII characters escaped. If ensure_ascii isfalse, these characters will be output as-is.

If check_circular is false (default: True), then the circularreference check for container types will be skipped and a circular referencewill result in a RecursionError (or worse).

If allow_nan is false (default: True), then it will be aValueError 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 andobject members will be pretty-printed with that indent level. An indent levelof 0, negative, or "" will only insert newlines. None (the default)selects the most compact representation. Using a positive integer indentindents that many spaces per level. If indent is a string (such as "\t"),that string is used to indent each level.

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 theircorresponding float values, which is outside the JSON spec.

object_hook, if specified, will be called with the result of every JSONobject decoded and its return value will be used in place of the givendict. This can be used to provide custom deserializations (e.g. tosupport JSON-RPC class hinting).

object_pairs_hook, if specified will be called with the result of everyJSON object decoded with an ordered list of pairs. The return value ofobject_pairs_hook will be used instead of the dict. Thisfeature can be used to implement custom decoders. If object_hook is alsodefined, the object_pairs_hook takes priority.

Changed in version 3.1: Added support for object_pairs_hook.

parse_constant, if specified, will be called with one of the followingstrings: '-Infinity', 'Infinity', 'NaN'.This can be used to raise an exception if invalid JSON numbersare encountered.

If strict is false (True is the default), then control characterswill be allowed inside strings. Control characters in this context arethose 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, aJSONDecodeError will be raised.

Python	JSON
dict	object
list, tuple	array
str	string
int, float, int- & float-derived Enums	number
True	true
False	false
None	null

Exceptions¶

exception json.JSONDecodeError(msg, doc, pos)¶

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.

Added in version 3.5.

Standard Compliance and Interoperability¶

The JSON format is specified by RFC 7159 and byECMA-404.This section details this module’s level of compliance with the RFC.For simplicity, JSONEncoder and JSONDecoder subclasses, andparameters other than those explicitly mentioned, are not considered.

This module does not comply with the RFC in a strict fashion, implementing someextensions 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 lastname-value pair is used.

Since the RFC permits RFC-compliant parsers to accept input texts that are notRFC-compliant, this module’s deserializer is technically RFC-compliant underdefault settings.

Character Encodings¶

The RFC requires that JSON be represented using either UTF-8, UTF-16, orUTF-32, with UTF-8 being the recommended default for maximum interoperability.

As permitted, though not required, by the RFC, this module’s serializer setsensure_ascii=True by default, thus escaping the output so that the resultingstrings only contain ASCII characters.

Other than the ensure_ascii parameter, this module is defined strictly interms of conversion between Python objects andUnicode strings, and thus does not otherwise directly addressthe 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 initialBOM in their input. This module’s deserializer raises a ValueErrorwhen an initial BOM is present.

The RFC does not explicitly forbid JSON strings which contain byte sequencesthat don’t correspond to valid Unicode characters (e.g. unpaired UTF-16surrogates), but it does note that they may cause interoperability problems.By default, this module accepts and outputs (when present in the originalstr) code points for such sequences.

Infinite and NaN Number Values¶

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 thisbehavior. In the deserializer, the parse_constant parameter can be used toalter this behavior.

Repeated Names Within an Object¶

The RFC specifies that the names within a JSON object should be unique, butdoes not mandate how repeated names in JSON objects should be handled. Bydefault, this module does not raise an exception; instead, it ignores all butthe 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.

Top-level Non-Object, Non-Array Values¶

The old version of JSON specified by the obsolete RFC 4627 required thatthe 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, andthis module does not and has never implemented that restriction in either itsserializer or its deserializer.

Regardless, for maximum interoperability, you may wish to voluntarily adhereto the restriction yourself.

Implementation Limitations¶

Some JSON deserializer implementations may set limits on:

the size of accepted JSON texts
the maximum level of nesting of JSON objects and arrays
the range and precision of JSON numbers
the content and maximum length of JSON strings

This module does not impose any such limits beyond those of the relevantPython datatypes themselves or the Python interpreter itself.

When serializing to JSON, beware any such limitations in applications that mayconsume your JSON. In particular, it is common for JSON numbers to bedeserialized into IEEE 754 double precision numbers and thus subject to thatrepresentation’s range and precision limitations. This is especially relevantwhen serializing Python int values of extremely large magnitude, orwhen serializing instances of “exotic” numerical types such asdecimal.Decimal.

Command Line Interface¶

Source code: Lib/json/tool.py

The json.tool module provides a simple command line interface to validateand pretty-print JSON objects.

If the optional infile and outfile arguments are notspecified, 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.toolExpecting 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 dictionariesalphabetically by key.

Command line options¶

infile¶

The 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.

outfile¶: Write the output of the infile to the given outfile. Otherwise, write itto sys.stdout.

--sort-keys¶

Sort the output of dictionaries alphabetically by key.

Added in version 3.5.

--no-ensure-ascii¶

Disable escaping of non-ascii characters, see json.dumps() for more information.

Added in version 3.9.

--json-lines¶

Parse every input line as separate JSON object.

Added in version 3.8.

--indent, --tab, --no-indent, --compact¶

Mutually exclusive options for whitespace control.

Added in version 3.9.

-h, --help¶: Show the help message.

Footnotes