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

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When you construct an object Python calls its __new__ method to create the object then calls __init__ on the object that is returned. When you create the object from inside __new__ by calling Triangle() that will result in further calls to __new__ and __init__.

What you should do is:

class Shape(object):
    def __new__(cls, desc):
        if cls is Shape:
            if desc == 'big':   return super(Shape, cls).__new__(Rectangle)
            if desc == 'small': return super(Shape, cls).__new__(Triangle)
        else:
            return super(Shape, cls).__new__(cls, desc)

which will create a Rectangle or Triangle without triggering a call to __init__ and then __init__ is called only once.

Edit to answer @Adrian’s question about how super works:

super(Shape,cls) searches cls.__mro__ to find Shape and then searches down the remainder of the sequence to find the attribute.

Triangle.__mro__ is (Triangle, Shape, object) and
Rectangle.__mro__ is (Rectangle, Shape, object) while Shape.__mro__ is just (Shape, object).
For any of those cases when you call super(Shape, cls) it ignores everything in the mro squence up to and including Shape so the only thing left is the single element tuple (object,) and that is used to find the desired attribute.

This would get more complicated if you had a diamond inheritance:

class A(object): pass
class B(A): pass
class C(A): pass
class D(B,C): pass

now a method in B might use super(B, cls) and if it were a B instance would search (A, object) but if you had a D instance the same call in B would search (C, A, object) because the D.__mro__ is (B, C, A, object).

So in this particular case you could define a new mixin class that modifies the construction behaviour of the shapes and you could have specialised triangles and rectangles inheriting from the existing ones but constructed differently.

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