Python module for JSON data encoding, including jsonlint. See the project Wiki here on Github. Also read the README at the bottom of this page, or the project homepage at
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dmeranda Fix jsonlint -f writing to stdout under Python 3.
Under Python3 the sys.stdout file object is open in text mode,
not binary mode.  So to write raw bytes to it (as is done when
using jsonlint's -f or -F options), you must use the underlying
raw file object (sys.stdout.buffer) that bypasses the text
mode wrapper.
Latest commit 5bc6597 Dec 22, 2015


demjson is a Python language module for encoding, decoding, and syntax-checking JSON data. It works under both Python 2 and Python 3.

It comes with a jsonlint script which can be used to validate your JSON documents for strict conformance to the JSON specification, and to detect potential data portability issues. It can also reformat or pretty-print JSON documents; either by re-indenting or removing unnecessary whitespace.

What's new

Version 2.2.4 fixes problem with jsonlint under Python 3 when trying to reformat JSON (-f or -F options) and writing the output to standard output.

Version 2.2.3 fixes incorrect return values from the "jsonlint" command. Also fixes a minor problem with the included unit tests in certain Python versions.

Version 2.2.2 fixes installation problems with certain Python 3 versions prior to Python 3.4. No other changes.

Version 2.2.1 adds an enhancement for HTML safety, and a few obscure bug fixes.

Version 2.2 fixes compatibility with Python 2.6 and narrow-Unicode Pythons, fixes bugs with statistics, and adds many enhancements to the treatment of numbers and floating-point values.

Version 2.0.1 is a re-packaging of 2.0, after discovering problems with incorrect checksums in the PyPI distribution of 2.0. No changes were made from 2.0.

Version 2.0, released 2014-05-21, is a MAJOR new version with many changes and improvements.

Visit for complete details and documentation. Additional documentation may also be found under the "docs/" folder of the source.

The biggest changes in 2.0 include:

  • Now works in Python 3; minimum version supported is Python 2.6
  • Much improved reporting of errors and warnings
  • Extensible with user-supplied hooks
  • Handles many additional Python data types automatically
  • Statistics

There are many more changes, as well as a small number of backwards incompatibilities. Where possible these incompatibilities were kept to a minimum, however it is highly recommended that you read the change notes thoroughly.

Example use

To use demjson from within your Python programs:

    >>> import demjson

    >>> demjson.encode( ['one',42,True,None] )    # From Python to JSON

    >>> demjson.decode( '["one",42,true,null]' )  # From JSON to Python
    ['one', 42, True, None]

To check a JSON data file for errors or problems:

    $ jsonlint my.json

    my.json:1:8: Error: Numbers may not have extra leading zeros: '017'
       |  At line 1, column 8, offset 8
    my.json:4:10: Warning: Object contains same key more than once: 'Name'
       |  At line 4, column 10, offset 49
       |  Object started at line 1, column 0, offset 0 (AT-START)
    my.json:9:11: Warning: Integers larger than 53-bits are not portable
       |  At line 9, column 11, offset 142
    my.json: has errors

Why use demjson?

I wrote demjson before Python had any JSON support in its standard library. If all you need is to be able to read or write JSON data, then you may wish to just use what's built into Python.

However demjson is extremely feature rich and is quite useful in certain applications. It is especially good at error checking JSON data and for being able to parse more of the JavaScript syntax than is permitted by strict JSON.

A few advantages of demjson are:

  • It works in old Python versions that don't have JSON built in;

  • It generally has better error handling and "lint" checking capabilities;

  • It will automatically use the Python Decimal (bigfloat) class instead of a floating-point number whenever there might be an overflow or loss of precision otherwise.

  • It can correctly deal with different Unicode encodings, including ASCII. It will automatically adapt when to use \u-escapes based on the encoding.

  • It generates more conservative JSON, such as escaping Unicode format control characters or line terminators, which should improve data portability.

  • In non-strict mode it can also deal with slightly non-conforming input that is more JavaScript than JSON (such as allowing comments).

  • It supports a broader set of Python types during conversion.


To install, type:

   python install

or optionally just copy the file "" to whereever you want. See "docs/INSTALL.txt" for more detailed instructions, including how to run the self-tests.

More information

See the files under the "docs" subdirectory. The module is also self-documented, so within the python interpreter type:

    import demjson

or from a shell command line:

    pydoc demjson

The "jsonlint" command script which gets installed as part of demjson has built-in usage instructions as well. Just type:

   jsonlint --help

Complete documentation and additional information is also available on the project homepage at

It is also available on the Python Package Index (PyPI) at


LGPLv3 - See the included "LICENSE.txt" file.

This software is Free Software and is licensed under the terms of the GNU LGPL (GNU Lesser General Public License). More information is found at the top of the source file and the included LICENSE.txt file.

Releases prior to 1.4 were released under a different license, be sure to check the corresponding LICENSE.txt file included with them.

This software was written by Deron Meranda,