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Python 3.6.5 Documentation > "json" — JSON encoder and decoder

"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.

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()"-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.

Changed in version 3.6: *s* can now be of type "bytes" or
"bytearray". The input encoding should be UTF-8, UTF-16 or UTF-32.

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
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_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.

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:

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.

Changed in version 3.2: Allow strings for *indent* in addition to
integers.

Changed in version 3.4: Use "(',', ': ')" as default if *indent* is
not "None".

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)

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*.

New in version 3.5.

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.

Character Encodings
-------------------

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.

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 this
behavior. In the deserializer, the *parse_constant* parameter can be
used to alter this behavior.

Repeated Names Within an Object
-------------------------------

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.

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

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.

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
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".

Command Line Interface
======================

**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.

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 it to "sys.stdout".

--sort-keys

Sort the output of dictionaries alphabetically by key.

New in version 3.5.

-h, --help

Show 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.

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