SudReg Web API's full documentation is online at Sudski registar api - javni korisnici.
pip install sudreg
from sudreg import SudskiRegistarAPI
api = SudskiRegistarAPI("Subscription-Key")
print(api.get_subjekt_detalji(tip_identifikatora="oib", identifikator="53056966535", expand_relations=True))