https://developer.openstack.org/api-ref/identity/v3/?expanded=assign-role-to-user-on-project-detail#assign-role-to-user-on-project
Assigning a user to project is currently not possible with the openstacksdk (identity v3). I think this should be a priority, because not being able to add users to a project seems to restrict the client in a lot of use cases. The API might should either be restrictive and project only or allow all the different use cases (projects, groups, domains and users).
IMO a Python 3 api that looks like:
def create_role_assignment(self, role, *, project=None, user=None, group=None, domain=None):
...
would do the job. Obviously this is a bit more complicated than the usual create_* methods. You need to use different urls (depending on the objects you have) and you also need to throw errors if a combination is not possible (which is true for a lot of the combos). You can also add different methods for all these use cases, but you will end up with a lot of combinations and that is not likely to decrease, rather the opposite.
Note that it's also not possible with the keystoneclient:
https://github.com/openstack/python-keystoneclient/blob/bca112c8ba5636becd6951fbfb8cb8f2474279fe/keystoneclient/v3/role_assignments.py#L107
~ Dave
Yeah we just went through a change to be able to list existing role assignments and it was quite involved. I'll try to take a look at this soon.