"""

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