descriptors.py

class StringProp(BaseDescriptor):
    required = False

    def __init__(self, default=None, required=False):
        self.default = default
        self.data = WeakKeyDictionary()
        self.required = bool(required)

    def __set__(self, instance, val):
        if self.required and not val:
            raise ValueError("Invalid value for required property: {0}".format(val))
        if not isinstance(val, basestring):
            raise ValueError("Invalid string property: {0}".format(val))
        self.data[instance] = val