Skip to main content

JSON serializer/deserializer for Python

Project description

Author: John Millikin
Copyright: This document has been placed in the public domain.


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.


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


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

>>> ('["Hello world!"]')
[u'Hello world!']


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

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


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

>>> print jsonlib.write (['Hello world!'], indent = '\t')
        "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):
jsonlib.errors.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 str, 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)



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 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.

Other JSON Libraries

demjson is a powerful and compliant library, which supports encoding autodetection, UTF-32, and surrogate pair handling. This is a very good library to use when extension modules cannot be installed. I advise always using “strict mode”.

simplejson is likely the most popular JSON library for Python, used by many web frameworks such as Django. Recent versions have improved in their support for Unicode, although it (as of 2008-03-28) still does not support encoding autodetection. I like demjson better.

python-cjson is designed for speed, but uses bytestrings internally and has poor support for Unicode. I advise against its use, unless you will only use ASCII text and need the performance.

python-json is one of the first JSON libraries for Python. It is no longer maintained and has numerous issues, especially regarding Unicode. I advise against using this library for any reason.

Change Log


  • Bugfix release, corrects serialization of dict when PyDict_Next() skips indexes.


  • Fixes an issue with reporting the column of a syntax error when the error is followed by a newline.
  • Removed remaining Python wrapper for read.


  • Support the on_unknown parameter to write.
  • Corrected typo in invalid whitespace detection.
  • Added __version__ attribute.
  • Merged all code into jsonlib and _jsonlib modules, instead of a package.


  • Improved the README.
  • Support for reading text encoded with the utf-8-sig codec.
  • Use codecs module for detecting BOMs in input data.
  • Forbid non-whitespace strings from being used for indentation.


  • Removed the Python implementations of the serializer and deserializer.
  • Detect and raise an exception if invalid surrogate pairs are serialized or deserialized.
  • Detect and raise an exception if reserved codepoints are serialized or deserialized.
  • Added support for operating in a process with multiple Python interpreters.
  • Performance improvements.


  • Allow python test to work.
  • Added encoding parameter to write, which controls the output encoding. The default encoding is utf-8. If the encoding is None, a unicode string will be returned.
  • Implemented write using a C extension module.


  • Improved error messages when an error is encountered deserializing an expression.
  • Modified to work with Python 2.4.


Thanks to Deron Meranda (author of demjson) for his excellent JSON library comparison, which revealed many areas for improvement:

  • Use repr instead of unicode for serializing floating-point values, to avoid unnecessary rounding.
  • Fixed bug that prevented plus signs in an exponent from being parsed correctly.
  • Added support for serializing the following types:
    • generator
    • set
    • frozenset
    • complex, for values with no imaginary component.
    • array.array
    • collections.deque
    • collections.defaultdict
    • UserList.UserList
    • UserDict.UserDict
    • UserString.UserString
  • Raise an exception if a control character is encountered in a string.
  • Added support for detecting Unicode byte order marks in the auto decoder.
  • Allow only arrays and objects to be serialized directly. All other types must be contained within an array or object.
  • Stricter detection of whitespace.

Also includes some other miscellaneous fixes:

  • More reliable detection of Infinity and NaN on Windows.
  • Support for decoding UTF-32 on UCS2 builds of Python.
  • Faster detection of self-recursive containers.


  • Return Unicode strings from write, so the user can control the final encoding.
  • Prevent Infinity, -Infinity, and NaN from being serialized because JSON does not support these values.
  • Added coerce_keys parameter to write. If True, mapping keys will be coerced to strings. Defaults to False.
  • Added ascii_only parameter to write. If True, non-ASCII codepoints will always be escaped to a u sequence. Defaults to True.
  • Real detection of self-recursive container types.
  • Escape the solidus to prevent against security issues.


  • Fixed bug that prevented characters from being read after reading a Unicode escape sequence.
  • Moved test cases into jsonlib.tests subpackage.


  • Port to setuptools.
  • Corrected false positive in detection of illegal leading zeroes.


  • Raise an exception if values in an object or array are not separated by commas.


  • Support for building on Windows.


  • Added sort_keys parameter to write. This allows mapping types to be serialized to a predictable value, regardless of key ordering.
  • Added indent to write. Any string passed as this value will be used for indentation. If the value is not None, pretty-printing will be activated.


  • Support for reading astral Unicode codepoints on UCS2 builds of Python.


  • Initial release.

Project details

Download files

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

Filename, size & hash SHA256 hash help File type Python version Upload date
jsonlib-1.3.5-py2.4-linux-i686.egg (39.6 kB) Copy SHA256 hash SHA256 Egg 2.4
jsonlib-1.3.5-py2.4-macosx-10.3-fat.egg (30.4 kB) Copy SHA256 hash SHA256 Egg 2.4
jsonlib-1.3.5-py2.4-win32.egg (17.8 kB) Copy SHA256 hash SHA256 Egg 2.4
jsonlib-1.3.5-py2.5-linux-i686.egg (39.7 kB) Copy SHA256 hash SHA256 Egg 2.5
jsonlib-1.3.5-py2.5-macosx-10.3-i386.egg (30.3 kB) Copy SHA256 hash SHA256 Egg 2.5
jsonlib-1.3.5-py2.5-win32.egg (17.9 kB) Copy SHA256 hash SHA256 Egg 2.5
jsonlib-1.3.5.tar.gz (18.6 kB) Copy SHA256 hash SHA256 Source None
jsonlib-1.3.5.win32-py2.4.exe (89.5 kB) Copy SHA256 hash SHA256 Windows Installer 2.4
jsonlib-1.3.5.win32-py2.5.exe (89.5 kB) Copy SHA256 hash SHA256 Windows Installer 2.5

Supported by

Elastic Elastic Search Pingdom Pingdom Monitoring Google Google BigQuery Sentry Sentry Error logging AWS AWS Cloud computing DataDog DataDog Monitoring Fastly Fastly CDN SignalFx SignalFx Supporter DigiCert DigiCert EV certificate StatusPage StatusPage Status page