Using a class' __new__ method as a Factory: __init__ gets called twice

Question

I encountered a strange bug in python where using the __new__ method of a class as a factory would lead to the __init__ method of the instantiated

Answer 1

After posting my question, I continued searching for a solution an found a way to solve the problem that looks like a bit of a hack. It is inferior to Duncan's solution, but I thought it could be interesting to mention none the less. The Shapeclass becomes:

class ShapeFactory(type):
    def __call__(cls, desc):
        if cls is Shape:
            if desc == 'big':   return Rectangle(desc)
            if desc == 'small': return Triangle(desc)
        return type.__call__(cls, desc)

class Shape(object):
    __metaclass__ = ShapeFactory 
    def __init__(self, desc):
        print "init called"
        self.desc = desc