Resolve conflicts file

Hi,
I'm trying to use pygit2 to resolve repo conflicts.
I intentionally created a conflict between a file in the local repo and a file in the remote repo. I have seen on certain methods that the favorite argument exists to have ours or theirs but I do not understand how to implement it, I always have "cannot create a tree from a not fully merged index" when I do my self .repo.index.write_tree()