+
−"""
+
−Convenience routines for creating non-trivial Field subclasses.
+
−
+
−Add SubfieldBase as the __metaclass__ for your Field subclass, implement
+
−to_python() and the other necessary methods and everything will work seamlessly.
+
−"""
+
−
+
−class SubfieldBase(type):
+
−    """
+
−    A metaclass for custom Field subclasses. This ensures the model's attribute
+
−    has the descriptor protocol attached to it.
+
−    """
+
−    def __new__(cls, base, name, attrs):
+
−        new_class = super(SubfieldBase, cls).__new__(cls, base, name, attrs)
+
−        new_class.contribute_to_class = make_contrib(
+
−                attrs.get('contribute_to_class'))
+
−        return new_class
+
−
+
−class Creator(object):
+
−    """
+
−    A placeholder class that provides a way to set the attribute on the model.
+
−    """
+
−    def __init__(self, field):
+
−        self.field = field
+
−
+
−    def __get__(self, obj, type=None):
+
−        if obj is None:
+
−            raise AttributeError('Can only be accessed via an instance.')
+
−        return obj.__dict__[self.field.name]        
+
−
+
−    def __set__(self, obj, value):
+
−        obj.__dict__[self.field.name] = self.field.to_python(value)
+
−
+
−def make_contrib(func=None):
+
−    """
+
−    Returns a suitable contribute_to_class() method for the Field subclass.
+
−
+
−    If 'func' is passed in, it is the existing contribute_to_class() method on
+
−    the subclass and it is called before anything else. It is assumed in this
+
−    case that the existing contribute_to_class() calls all the necessary
+
−    superclass methods.
+
−    """
+
−    def contribute_to_class(self, cls, name):
+
−        if func:
+
−            func(self, cls, name)
+
−        else:
+
−            super(self.__class__, self).contribute_to_class(cls, name)
+
−        setattr(cls, self.name, Creator(self))
+
−
+
−    return contribute_to_class