# `!json` --- JSON encoder and decoder
json
**Source code:** `Lib/json/__init__.py`
------------------------------------------------------------------------
[JSON (JavaScript Object Notation)](https://json.org), specified by `7159` (which obsoletes `4627`) and by [ECMA-404](https://ecma-international.org/publications-and-standards/standards/ecma-404/), is a lightweight data interchange format inspired by [JavaScript](https://en.wikipedia.org/wiki/JavaScript) object literal syntax (although it is not a strict subset of JavaScript[^1] ).
> [!NOTE]
> The term "object" in the context of JSON processing in Python can be ambiguous. All values in Python are objects. In JSON, an object refers to any data wrapped in curly braces, similar to a Python dictionary.
> [!WARNING]
> Be cautious when parsing JSON data from untrusted sources. A malicious JSON string may cause the decoder to consume considerable CPU and memory resources. Limiting the size of data to be parsed is recommended.
This module 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({'6': 7, '4': 5}, sort_keys=True, indent=4))
{
"4": 5,
"6": 7
}
Customizing JSON object encoding:
>>> import json
>>> def custom_json(obj):
... if isinstance(obj, complex):
... return {'__complex__': True, 'real': obj.real, 'imag': obj.imag}
... raise TypeError(f'Cannot serialize object of {type(obj)}')
...
>>> json.dumps(1 + 2j, default=custom_json)
'{"__complex__": true, "real": 1.0, "imag": 2.0}'
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']
Customizing 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')
>>> json.loads('{"name": "point", "coords": [[1, 2], [3, 4]]}',
... array_hook=tuple) # doctest: +SKIP
{'name': 'point', 'coords': ((1, 2), (3, 4))}
>>> json.loads('{"name": "point", "coords": [1, 2]}',
... object_pairs_hook=frozendict, array_hook=tuple) # doctest: +SKIP
frozendict({'name': 'point', 'coords': (1, 2)})
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` from the shell to validate and pretty-print:
```
$ echo '{"json":"obj"}' | python -m json
{
"json": "obj"
}
$ echo '{1.2:3.4}' | python -m json
Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
```
See `json-commandline` for detailed documentation.
> [!NOTE]
> JSON is a subset of [YAML](https://yaml.org/) 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.
> [!NOTE]
> This module's encoders and decoders preserve input and output order by default. Order is only lost if the underlying containers are unordered.
## Basic Usage
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 `Python-to-JSON conversion table
`.
> [!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.
param object obj
The Python object to be serialized.
param fp
The file-like object *obj* will be serialized to. The `!json` module always produces `str` objects, not `bytes` objects, therefore `fp.write()` must support `str` input.
type fp
`file-like object`
param bool skipkeys
If `True`, keys that are not of a basic type (`str`, `int`, `float`, `bool`, `None`) will be skipped instead of raising a `TypeError`. Default `False`.
param bool ensure_ascii
If `True` (the default), the output is guaranteed to have all incoming non-ASCII and non-printable characters escaped. If `False`, all characters will be outputted as-is, except for the characters that must be escaped: quotation mark, reverse solidus, and the control characters U+0000 through U+001F.
param bool check_circular
If `False`, the circular reference check for container types is skipped and a circular reference will result in a `RecursionError` (or worse). Default `True`.
param bool allow_nan
If `False`, serialization of out-of-range `float` values (`nan`, `inf`, `-inf`) will result in a `ValueError`, in strict compliance with the JSON specification. If `True` (the default), their JavaScript equivalents (`NaN`, `Infinity`, `-Infinity`) are used.
param cls
If set, a custom JSON encoder with the `~JSONEncoder.default` method overridden, for serializing into custom datatypes. If `None` (the default), `!JSONEncoder` is used.
type cls
a `JSONEncoder` subclass
param indent
If a positive integer or string, JSON array elements and object members will be pretty-printed with that indent level. A positive integer indents that many spaces per level; a string (such as `"\t"`) is used to indent each level. If zero, negative, or `""` (the empty string), only newlines are inserted. If `None` (the default), no newlines are inserted.
type indent
int \| str \| None
param separators
A two-tuple: `(item_separator, key_separator)`. If `None` (the default), *separators* defaults to `(', ', ': ')` if *indent* is `None`, and `(',', ': ')` otherwise. For the most compact JSON, specify `(',', ':')` to eliminate whitespace.
type separators
tuple \| None
param default
A function that is called for objects that can't otherwise be serialized. It should return a JSON encodable version of the object or raise a `TypeError`. If `None` (the default), `!TypeError` is raised.
type default
`callable` \| None
param bool sort_keys
If `True`, dictionaries will be outputted sorted by key. Default `False`.
> [!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 converted 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. *sort_keys* sorts the keys before they are converted to strings, so numeric keys are sorted by value, not by their string representation.
3.2 Allow strings for *indent* in addition to integers.
3.4 Use `(',', ': ')` as default if *indent* is not `None`.
3.6 All optional parameters are now `keyword-only `.
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`.
load(fp, *, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, array_hook=None,*\*kw)
Deserialize *fp* to a Python object using the `JSON-to-Python conversion table
`.
param fp
A `.read()`-supporting `text file` or `binary file` containing the JSON document to be deserialized.
type fp
`file-like object`
param cls
If set, a custom JSON decoder. Additional keyword arguments to `!load` will be passed to the constructor of *cls*. If `None` (the default), `!JSONDecoder` is used.
type cls
a `JSONDecoder` subclass
param object_hook
If set, a function that is called with the result of any JSON object literal decoded (a `dict`). The return value of this function will be used instead of the `dict`. This feature can be used to implement custom decoders, for example [JSON-RPC](https://www.jsonrpc.org) class hinting. Default `None`.
type object_hook
`callable` \| None
param object_pairs_hook
If set, a function that is called with the result of any JSON object literal decoded with an ordered list of pairs. The return value of this function will be used instead of the `dict`. This feature can be used to implement custom decoders. If *object_hook* is also set, *object_pairs_hook* takes priority. Default `None`.
type object_pairs_hook
`callable` \| None
param array_hook
If set, a function that is called with the result of any JSON array literal decoded as a Python list. The return value of this function will be used instead of the `list`. This feature can be used to implement custom decoders. Default `None`.
type array_hook
`callable` \| None
param parse_float
If set, a function that is called with the string of every JSON float to be decoded. If `None` (the default), it is equivalent to `float(num_str)`. This can be used to parse JSON floats into custom datatypes, for example `decimal.Decimal`.
type parse_float
`callable` \| None
param parse_int
If set, a function that is called with the string of every JSON int to be decoded. If `None` (the default), it is equivalent to `int(num_str)`. This can be used to parse JSON integers into custom datatypes, for example `float`.
type parse_int
`callable` \| None
param parse_constant
If set, a function that is called with one of the following strings: `'-Infinity'`, `'Infinity'`, or `'NaN'`. This can be used to raise an exception if invalid JSON numbers are encountered. Default `None`.
type parse_constant
`callable` \| None
raises JSONDecodeError
When the data being deserialized is not a valid JSON document.
raises UnicodeDecodeError
When the data being deserialized does not contain UTF-8, UTF-16 or UTF-32 encoded data.
3.1
- Added the optional *object_pairs_hook* parameter.
- *parse_constant* doesn't get called on 'null', 'true', 'false' anymore.
3.6
- All optional parameters are now `keyword-only `.
- *fp* can now be a `binary file`. The input encoding should be UTF-8, UTF-16 or UTF-32.
3.11 The default *parse_int* of `int` now limits the maximum length of the integer string via the interpreter's `integer string
conversion length limitation ` to help avoid denial of service attacks.
3.15 Added the optional *array_hook* parameter.
loads(s, *, cls=None, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, object_pairs_hook=None, array_hook=None,*\*kw)
Identical to `load`, but instead of a file-like object, deserialize *s* (a `str`, `bytes` or `bytearray` instance containing a JSON document) to a Python object using this `conversion table
`.
3.6 *s* can now be of type `bytes` or `bytearray`. The input encoding should be UTF-8, UTF-16 or UTF-32.
3.9 The keyword argument *encoding* has been removed.
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* is an optional function that 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](https://www.jsonrpc.org) class hinting).
*object_pairs_hook* is an optional function that 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. If *object_hook* is also defined, the *object_pairs_hook* takes priority.
3.1 Added support for *object_pairs_hook*.
*array_hook* is an optional function that will be called with the result of every JSON array decoded as a list. The return value of *array_hook* will be used instead of the `list`. This feature can be used to implement custom decoders.
3.15 Added support for *array_hook*.
*parse_float* is an optional function that 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* is an optional function that 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* is an optional function that 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.
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.
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.
Extensible JSON encoder for Python data structures.
Supports the following objects and types by default:
| Python | JSON |
|----------------------------------------|--------|
| dict, frozendict | object |
| list, tuple | array |
| str | string |
| int, float, int- & float-derived Enums | number |
| True | true |
| False | false |
| None | null |
3.4 Added support for int- and float-derived Enum classes.
3.15 Added support for `frozendict`.
To extend this to recognize other objects, subclass and implement a `~JSONEncoder.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), a `TypeError` will be raised when trying to encode keys that are not `str`, `int`, `float`, `bool` 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 and non-printable characters escaped. If *ensure_ascii* is false, all characters will be output as-is, except for the characters that must be escaped: quotation mark, reverse solidus, and the control characters U+0000 through U+001F.
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 a `RecursionError`). 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.
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.
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.
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 `~JSONEncoder.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 super().default(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)
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*.
3.5
json.tool
**Source code:** `Lib/json/tool.py`
------------------------------------------------------------------------
The `!json` module can be invoked as a script via `python -m json` to validate and pretty-print JSON objects. The `!json.tool` submodule implements this interface.
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
{
"json": "obj"
}
$ echo '{1.2:3.4}' | python -m json
Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
```
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.
3.14 The `!json` module may now be directly executed as `python -m json`. For backwards compatibility, invoking the CLI as `python -m json.tool` remains supported.
### Command-line options
infile
The JSON file to be validated or pretty-printed:
```
$ python -m json 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 it to `sys.stdout`.
`--sort-keys`
Sort the output of dictionaries alphabetically by key.
3.5
`--no-ensure-ascii`
Disable escaping of non-ascii characters, see `json.dumps` for more information.
3.9
`--json-lines`
Parse every input line as separate JSON object.
3.8
`--indent, --tab, --no-indent, --compact`
Mutually exclusive options for whitespace control.
3.9
`-h, --help`
Show the help message.
**Footnotes**
[^1]: As noted in [the errata for RFC 7159](https://www.rfc-editor.org/errata_search.php?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.