def get_source_fields(self):
        return [e._output_field_or_none for e in self.get_source_expressions()]
Hook called before spawning a user's server.
dataset/ETHPy150Open django/django/django/db/models/expressions.py/BaseExpression.get_source_fields--crossover with--dataset/ETHPy150Open jupyterhub/jupyterhub/jupyterhub/auth.py/Authenticator.pre_spawn_start