Flask-Multipass provides Flask with a user authentication/identity system which can use different backends (such as local users, LDAP and OAuth) simultaneously.
It was developed at CERN and is currently used in production by Indico.
There are bult-in authentication and identity providers for:
Those can be used simultaneously and interchangeably (e.g. authenticate with OAuth and search users with LDAP).
Documentation is available at https://flask-multipass.readthedocs.org