I created an initial package to run prosemirror transformations in Python code [1] with a simple test of applying a step to a doc that had been loaded previously and then turning the doc back into JSON [2].
Translating the relevant parts of prosemirror-model and prosemirror-transform to Python code using Js2Py [3] seems to be working.
I have put it under the AGPL license so far, as I’m not really interested in maintaining it if other closed-source projects are going to use it without giving back, but I would be open to change the license to something else if someone else is willing to take on maintenance, writing tests, expanding on it, etc. .
The advantage of auto-translating the JavaScript to Python code like this is of course that it will always be in sync with prosemirror itself and there should be no room for weird edge cases, inconsistencies, etc. exept possible shortcomings in Js2Py.
[1] https://github.com/fiduswriter/prosemirror-python/
[2] https://github.com/fiduswriter/prosemirror-python/blob/master/test.py