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I think it's more of a limitation. I seems to me the main
focus in implementing pprint was speed not extensibility.
The code uses every trick in the book (e.g. turning globals
and builtins into locals, using bound methods etc.). I think
it was never ment to do anything other than what repr()
does, but with better formatting. However IMHO making pprint
extensible would be a worthwile project.