examples of python class customization, generic descriptors

p2.py

import math

class Magnitude(object):
    '''A descriptor that can be added to any N-dimensional point to provide a read/writable magnitude property.'''

    def __init__(self, *attributes):
        self.attributes = attributes
        
    def __get__(self, instance, owner):
        sum_of_squares = 0.0
        for attr in self.attributes:
            value = getattr(instance, attr, 0)
            sum_of_squares += value * value
        return math.sqrt(sum_of_squares)

    def __set__(self, instance, new_magnitude):
        current_magnitude = Magnitude.__get__(self, instance, instance.__class__)
        if current_magnitude:
            scale_factor = float(new_magnitude) / current_magnitude
            for attr in self.attributes:
                value = getattr(instance, attr, 0)
                setattr(instance, attr, value * scale_factor)
        else:
            # can't scale a zero vector...
            pass


class P2(object):
    '''A 2D point class providing many examples of Python class customization.'''

    __slots__ = ('x', 'y')
    	
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __str__(self):
        return 'P2({}, {})'.format(self.x, self.y)

    def __unicode__(self):
        return unicode(self.__str__())

    def __repr__(self):
        return self.__str__()

    def __getitem__(self, index):
        if index == 0 or index == 'x':
            return self.x
        elif index == 1 or index == 'y':
            return self.y
        else:
            raise IndexError('point index out of range')

    def __setitem__(self, index, value):
        if index == 0 or index == 'x':
            self.x = value
        elif index == 1 or index == 'y':
            self.y = value
        else:
            raise IndexError('point index out of range')

    def __len__(self):
        return 2

    def __iter__(self):
        yield self.x
        yield self.y

    def __complex__(self):
        return complex(self.x, self.y)

    def __add__(self, other):
        return self.__class__(self.x + other.x, self.y + other.y)

    magnitude = Magnitude('x', 'y')



p = P2(3,4)
print p
print p.magnitude

# normalize the vector
p.magnitude = 1
print p

p[0] = 4
p[1] = 3
print p

print 'length:', len(p)

x, y = p
print 'unpacked:', x, y

for v in p:
    print v

if 4 in p: print '4 in p'
if 5 in p: print '5 in p'

print complex(p)

print p + P2(10, 10)