Right now, json.dumps can be called with True or False, but it would be easy to also support accepting a key function, which then could be used to control the order of keys arbitrarily in the serialized JSON output.

Implementation for the enhancement.  Includes tests.

The idea and patch look reasonable.

Bob, what do you think?

Seems like a good idea to me, I'll make sure this gets in simplejson as well.

On further investigation, simplejson has implemented this functionality under a different name since 2.5.0 (2012-03-29).

"""
If item_sort_key is a callable (not the default), then the output of dictionaries will be sorted with it. The callable will be used like this: sorted(dct.items(), key=item_sort_key). This option takes precedence over sort_keys.

Changed in version 2.5.0: item_sort_key is new in 2.5.0.
"""

Catherine, if you are going to continue contributing patches, which we hope you do, please sign the PSF Contribution Agreement.
https://www.python.org/psf/contrib/
https://www.python.org/psf/contrib/contrib_form/

It would be needed if this patch were to be used, but I presume Bob would import the patch from simplejson instead.

As there was interest in this patch, @catherinedevlin would you be able to make a Github pull request for it?

Thanks!

I've rebased Catherine Devlin's patch onto 3.9 (and adjusted the docs a tiny bit). If anyone still is interested, I can create a PR (for 3.9) for this.