Documented here.

class ReadOnlyDescriptor:

def __set_name__(self, owner, name):
    self.private_name = "_" + name

def __get__(self, obj, objtype=None):
    return getattr(obj, self.private_name)

def __set__(self, obj, value):
    raise AttributeError("Cannot set this!")

def __delete__(self, obj):
    raise AttributeError("Cannot delete this!")