Descriptors, __getattr__

attribute_acess_first.py

# http://nbviewer.ipython.org/urls/gist.github.com/ChrisBeaumont/5758381/raw/descriptor_writeup.ipynb
# http://pyvideo.org/video/1760/encapsulation-with-descriptors


class NonNegativeValue(object):

    __counter = 0

    def __init__(self):
        prefix = self.__class__.__name__
        key = self.__class__.__counter
        self.target_name = "_{0}_{1}".format(prefix, str(key))
        self.__class__.__counter += 1

        print 'Target name', self.target_name

    def __get__(self, instance, owner):
        return getattr(instance, self.target_name)

    def __set__(self, instance, value):

        if value <= 0:
            raise ValueError('value must be > 0')
        setattr(instance, self.target_name, value)


class LineItem(object):
    weight = NonNegativeValue()
    price = NonNegativeValue()

    print "inside lineitem "

    def __init__(self, description, weight, price):
        self.description = description
        self.weight = weight
        self.price = price

    def __getattr__(self, item):

        print '__getattr__ called on self with item {0}'.format(item)

        if item == 'blah':
            return self.__dict__.get(item, 'default_blah')

        raise AttributeError("{0} has no attribute {1}").format(
            self.__class__.__name__, item
        )

    def __setattr__(self, key, value):

        print '__setattr__ called on self with key:{0}, value:{1}'.format(
            str(key), str(value)
        )
        # http://docs.python.org/2/reference/datamodel.html#implementing-descriptors
        # self.__dict__[key] = value
        super(LineItem, self).__setattr__(key, value)

    def subtotal(self):
        return self.weight * self.price
        
        
rice = LineItem('rice', 3, 5.65)
milk = LineItem('milk', 2, 4.50)
almonds = LineItem('almonds', 10, 14.95)