Here’s a first draft at a patch.  I’ve added a module-level print function and a PrettyPrinter.print method which does the real work.

The functions have a signature compatible with print, but I have changed one default value: sep defaults to ',\n', because I think that the output is more useful and more pleasing (otherwise you’d have strange-looking indentation with containers).

If this looks good, I’ll work on doc and tests.  I guess I’ll reuse the helpers in test_print.