Skip to main content

JSON serializer/deserializer for Python

Project description


John Millikin


JSON is a lightweight data-interchange format. It is often used for exchanging data between a web server and user agent.

This module aims to produce a library for serializing and deserializing JSON that conforms strictly to RFC 4627.

For the Python 3 version of jsonlib, see jsonlib-python3.

Other JSON implementations of interest include simplejson (available in the standard library as of Python 2.6) and demjson.


jsonlib has two functions of interest, read and write. It also defines some exception: ReadError, WriteError, and UnknownSerializerError.

For compatibility with the standard library, read is aliased to loads and write is aliased to dumps. They do not have the same set of advanced parameters, but may be used interchangeably for simple invocations.


To deserialize a JSON expression, call the function with an instance of unicode or bytes.

>>> import jsonlib
>>> (b'["Hello world!"]')
[u'Hello world!']

Floating-point values

By default, jsonlib will parse values such as “1.1” into an instance of decimal.Decimal. To use the built-in value type float instead, set the use_float parameter to True. float values are much faster to construct, so this flag may substantially increase parser performance.

Please note that using float will cause a loss of precision when parsing some values.

>>> ('[3.14159265358979323846]', use_float = True)


Serialization has more options, but they are set to reasonable defaults. The simplest use is to call jsonlib.write with a Python value.

>>> import jsonlib
>>> jsonlib.write (['Hello world!'])
'["Hello world!"]'


To “pretty-print” the output, pass a value for the indent parameter.

>>> print (jsonlib.write (['Hello world!'], indent = '    ').decode ('utf8'))
    "Hello world!"

Mapping Key Sorting

By default, mapping keys are serialized in whatever order they are stored by Python. To force a consistent ordering (for example, in doctests) use the sort_keys parameter.

>>> jsonlib.write ({'e': 'Hello', 'm': 'World!'})
>>> jsonlib.write ({'e': 'Hello', 'm': 'World!'}, sort_keys = True)

Encoding and Unicode

By default, the output is encoded in UTF-8. If you require a different encoding, pass the name of a Python codec as the encoding parameter.

>>> jsonlib.write (['Hello world!'], encoding = 'utf-16-be')
'\x00[\x00"\x00H\x00e\x00l\x00l\x00o\x00 \x00w\x00o\x00r\x00l\x00d\x00!\x00"\x00]'

To retrieve an unencoded unicode instance, pass None for the encoding.

>>> jsonlib.write (['Hello world!'], encoding = None)
u'["Hello world!"]'

By default, non-ASCII codepoints are forbidden in the output. To include higher codepoints in the output, set ascii_only to False.

>>> jsonlib.write ([u'Hello \u266a'], encoding = None)
u'["Hello \\u266a"]'
>>> jsonlib.write ([u'Hello \u266a'], encoding = None, ascii_only = False)
u'["Hello \u266a"]'

Mapping Key Coercion

Because JSON objects must have string keys, an exception will be raised when non-string keys are encountered in a mapping. It can be useful to coerce mapping keys to strings, so the coerce_keys parameter is available.

>>> jsonlib.write ({True: 1})
Traceback (most recent call last):
WriteError: Only strings may be used as object keys.
>>> jsonlib.write ({True: 1}, coerce_keys = True)

Serializing Other Types

If the object implements the iterator or mapping protocol, it will be handled automatically. If the object is intended for use as a basic value, it should subclass one of the supported basic values.

String-like objects that do not inherit from unicode or UserString.UserString will likely be serialized as a list. This will not be changed. If iterating them returns an instance of the same type, the serializer might crash. This (hopefully) will be changed.

To serialize a type not known to jsonlib, use the on_unknown parameter to write:

>>> from datetime import date
>>> def unknown_handler (value):
...     if isinstance (value, date):
...         return str (value)
...     raise jsonlib.UnknownSerializerError
>>> jsonlib.write ([date (2000, 1, 1)], on_unknown = unknown_handler)

Streaming Serializer

When serializing large objects, the use of an in-memory buffer may cause too much memory to be used. For these situations, use the dump function to write objects to a file-like object:

>>> import sys
>>> jsonlib.dump (["Written to stdout"], sys.stdout, encoding = None)
["Written to stdout"]
>>> with open ("/dev/null", "wb") as out:
...     jsonlib.dump (["Written to a file"], out)



Raised by read if an error was encountered parsing the expression. Will contain the line, column, and character position of the error.

Note that this will report the character, not the byte, of the character that caused the error.


Raised by write or dump if an error was encountered serializing the passed value.


A subclass of WriteError that is raised when a value cannot be serialized. See the on_unknown parameter to write.

Change Log


  • Fixed error in write() which could cause output truncation.


  • Performance improvements

  • coerce_keys no longer attempts to determine the “JSON” format for a coerced value – it will simply call unicode().

Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

jsonlib-1.6.1.tar.gz (43.8 kB view hashes)

Uploaded Source

Supported by

AWS AWS Cloud computing and Security Sponsor Datadog Datadog Monitoring Fastly Fastly CDN Google Google Download Analytics Microsoft Microsoft PSF Sponsor Pingdom Pingdom Monitoring Sentry Sentry Error logging StatusPage StatusPage Status page