`json` and `simplejson` module differences in Python

json is simplejson, added to the stdlib. Since json was only added in 2.6, simplejson has the advantage of working on more python versions (2.4+, rather than 2.6+). Also, simplejson is updated more frequently than Python is, so if you need the latest version for some reason, it's best to use simplejson itself.

A good practice, in my opinion, is to use it as a fallback.

try: import json
except ImportError: import simplejson as json

The builtin json module got included in Python 2.6. Any projects that support versions of Python < 2.6 need to have a fallback. In many cases, that fallback is simplejson.

Here's (a now outdated) comparison of Python json libraries:

Comparing JSON modules for Python

Regardless of the results in this comparison you should use the standard library json if you are on Python 2.6. And.. might as well just use simplejson otherwise.

Another reason projects use simplejson is that the builtin json did not include its C speedups, so the performance difference is noticeable.