In Python 2.6.4, json.dumps(...,indent=0) produced newlines as documented here: http://docs.python.org/library/json.html#json.dump
In Python 2.7, it no longer adds newlines.

$ python
Python 2.6.4 (r264:75706, Jan 13 2010, 19:41:08) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> json.dumps({3:1,4:2},indent=0)
'{\n"3": 1, \n"4": 2\n}'
>>> print json.dumps({3:1,4:2},indent=0)
{
"3": 1, 
"4": 2
}



$ python
Python 2.7 (r27:82500, Oct  3 2010, 06:00:45) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> json.dumps({3:1,4:2})
'{"3": 1, "4": 2}'
>>> print json.dumps({3:1,4:2},indent=0)
{"3": 1, "4": 2}

The C encoder should not be used when indent=0. Here is a patch+test for 2.7.
Note that json.dump (into a file object) already correctly emit newlines.

The test in the patch assumes a specific iteration order for the dict "h", changing the dict to have only one key would fix this problem with the test.

The test also repeats an equivalent dict to "h" in the check function.

I just applied a version of this patch with corrections to the tests here:
http://code.google.com/p/simplejson/source/detail?r=234

+1 to merge simplejson 2.1+ before 3.2 beta 1 (mid-november)

Hi,
I updated the patch, making one for 2.7, 3.1 and 3.2 (this last one applies cleanly on default too).

As of merging simplejson, it's more a matter of porting it to Python 3. I'll drop an email to Bob soon, let's see how it goes.

Oh, just to say I took the version as of http://code.google.com/p/simplejson/source/detail?r=234

New changeset 8264f68e8251 by R David Murray in branch '2.7':
#10019: Fix regression relative to 2.6: add newlines if indent=0
http://hg.python.org/cpython/rev/8264f68e8251

New changeset 4a1048257995 by R David Murray in branch '3.1':
#10019: Fix regression relative to 2.6: add newlines if indent=0
http://hg.python.org/cpython/rev/4a1048257995

New changeset fe8bbaff5a27 by R David Murray in branch '3.2':
Merge #10019: Fix regression relative to 2.6: add newlines if indent=0
http://hg.python.org/cpython/rev/fe8bbaff5a27

New changeset 2d0d0850335e by R David Murray in branch 'default':
Merge #10019: Fix regression relative to 2.6: add newlines if indent=0
http://hg.python.org/cpython/rev/2d0d0850335e

Thanks, Sandro.