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The json "sort_keys" parameter is actually supported as an argument to dump() and dumps() (and is used that way in the examples), but is only documented as an argument to the JSONEncoder constructor.

In json module there are dump/dumps methods which internally instantiate encoder class JSONEncoder (or some other user-defined encoder clas).
They look as follows:
json.dump(obj, fp, 
 skipkeys=False,
 ensure_ascii=True,
 check_circular=True, 
 allow_nan=True,
 cls=None,
 indent=None,
 separators=None,
 default=None, 
 **kw)
json.JSONEncoder(skipkeys=False, 
 ensure_ascii=True,
 check_circular=True,
 allow_nan=True,
 sort_keys=False,
 indent=None,
 separators=None,
 default=None)
Some of dump/dumps arguments are passed to encoder class:
- skipkeys
- ensure_ascii
- check_circular
- allow_nan
- indent
- separators
- default
And it looks that sort_keys is just missing in keyword args in dump/dumps method.
But it still can be passed implicitly using **kw arg.
I would propose to do:
- add explicitly sort_keys keyword arg to dump/dumps methods
- add passing it to encoder class
- and adjust documentation accordingly.

Attached a patch.
To preserve current order of arguments in dumps/dump functions sort_keys argument has been added to the end of arguments just before **kw.

Erm, the patch seems to have some ANSI terminal coloring escape code junk (e.g. "") in it...

Attached updated patches.

Any reactions? The patch is pretty straightforward...

I would recommend making the added and modified lines not exceed 79 characters in both files. Also, for the .rst file, you can use slashes to break lines as shown in the following example:
http://hg.python.org/cpython/file/09011896374d/Doc/library/argparse.rst#l133 

Also, while not strictly necessary, it is more customary and convenient to provide a single patch file for all files.

Per Chris Jerdonek, here's a combined patch that also makes the docs changes comply with the antiquated line length limit.

New changeset 40aedc7da30f by Andrew Svetlov in branch '3.2':
Issue #14570: Document json sort_keys parameter properly.
http://hg.python.org/cpython/rev/40aedc7da30f
New changeset 090484ccba7d by Andrew Svetlov in branch '3.3':
Merge issue #14570: Document json sort_keys parameter properly.
http://hg.python.org/cpython/rev/090484ccba7d
New changeset 27ce005372a5 by Andrew Svetlov in branch 'default':
Merge issue #14570: Document json sort_keys parameter properly.
http://hg.python.org/cpython/rev/27ce005372a5 

Thanks, Chris!
I have pushed the patch for 3.2-3.4
It cannot be applied to 2.7, please make separate patch for this version.

Patch for 2.7. This assumes that changing the parameter notation is permissible.

New changeset 7784008e9ade by Andrew Svetlov in branch '2.7':
Issue #14570: Document json sort_keys parameter properly.
http://hg.python.org/cpython/rev/7784008e9ade 

Fixed, thanks again!