        def _class_str(obj):
            """Given an instance, return the 'module.Class' string"""
            if obj is None:
                return None

            if obj.__class__ is type:
                cls = obj
            else:
                cls = obj.__class__
            return f"{cls.__module__}.{cls.__name__}"
Given an instance, return the 'module.Class' string
jupyterhub/tests/extension/__init__.py