(Python) __new__ method returning something other than class instance

Question

Is it possible, and if it is, then how to do so, that creating a new object of a class returns something other than the object itself?

Suppose, I want every newly created object to start out as a list consisting of itself.

>> class A:
      *magic*

>> a = A()
>> print a

[<__main__.A instance at 0x01234567>]

Probably it can be done by overriding the __new__ method in some way, but how?

Answer 1

Yes, __new__ can return something else than the new instance:

class A(object):
    def __new__(cls, *args, **kw):
        instance = super(A, cls).__new__(cls, *args, **kw)
        return [instance]

Demo:

>>> class A(object):
...     def __new__(cls, *args, **kw):
...         instance = super(A, cls).__new__(cls, *args, **kw)
...         return [instance]
... 
>>> A()
[<__main__.A object at 0x10f33f390>]

Note that __init__ is not called if the returned object is not the same type (so isinstance(returned_value, A) is False); that means that a custom A.__init__() is never going to be called here! Call it explicitly:

class A(object):
    def __new__(cls, *args, **kw):
        instance = super(A, cls).__new__(cls, *args, **kw)
        if hasattr(cls, '__init__'):
            instance.__init__(*args, **kw)            
        return [instance]