hacky_factory.py

class Shape(object):
  def __new__(cls, desc, color='black'):
      if cls is Shape:
        if desc == 'sankaku':
          return super(Shape, cls).__new__(Triangle, color)
        elif desc == 'shikaku':
          return super(Shape, cls).__new__(Rectangle, color)
        else:
          return super(Shape, cls).__new__(Shape, color)
      else:
        return super(Shape, cls).__new__(cls, desc, color)

  def __init__(self, desc, color='black'):
    print "__init__ is called. desc = %s. color = %s" % (desc, color)
    self.desc = desc

  @property
  def number_of_edges(self): return 0


class Triangle(Shape):
  @property
  def number_of_edges(self): return 3


class Rectangle(Shape):
  @property
  def number_of_edges(self): return 4


instance1 = Shape('sankaku')
print 'number_of_edges = %d' % instance1.number_of_edges
print type(instance1)

print ''

instance2 = Shape('shikaku', 'red')
print 'number_of_edges = %d' % instance2.number_of_edges
print type(instance2)

print ''

instance3 = Shape('foo', 'blue')
print 'number_of_edges = %d' % instance3.number_of_edges
print type(instance3)

init is called. desc = sankaku. color = black
number_of_edges = 3
class 'main.Triangle'

init is called. desc = shikaku. color = red
number_of_edges = 4
class 'main.Rectangle'

init is called. desc = foo. color = blue
number_of_edges = 0
class 'main.Shape'