Descriptor not seen as class in Sphinx

descriptor_docstring.py

class ValueFilter(object):
    """A descriptor which applies *filter* to assigned values, which are stored
    at *attr*.  If the value is unset it is *default*.
    """
    def __init__(self, attr, filter, default=None):
        self.attr = attr
        self.filter = filter
        self.default = default

    def __get__(self, instance, owner):
        if instance is None:
            return getattr(owner, self.attr, self.default)
        else:
            return getattr(instance, self.attr, self.default)

    def __set__(self, instance, value):
        setattr(instance, self.attr, self.filter(value))