clever... shows mastery of language features and idioms.
please never allow this into production code.
to improve this, i would suggest...
either add a long explanation of how this solves the problem
at hand, or (preferably) simplify and make the logic more self explanatory, naming vars meaningfully to convey intent, injecting sparse comments to explain why (why are you doing these things in yr code?).
if you do these things, the person that inherits your code when you move on to greener pastures
will NOT curse you under their breath, but rather, will have only the highest praise for your professionalism and coding skills.

My bad. I totally missed the whole recursive part of the digit summing process.
Please accept my sincere apologies.

Although unnecessary work in Python, I think this is a better solution for languages that do not support 10000 digit integers.

Javascript, Python (probably other languages) need random tests.

What error do you get? It seems fine to me (just tested it). Read the instructions again and return a single digit number.

absolutely not.

This kata is broken for Python 3