Question regarding __new__

[Frank Benkstein]

Hi,

the behaviour I always observed when creating instances by calling the
class A is that '__init__' is always only called when the object
returned by A.__new__ is an instance of A. This can be observed by the
following code:

class A(object):
    def __new__(cls, *args, **kwds):
        print "A.__new__", args, kwds
        return object.__new__(B, *args, **kwds)
    def __init__(cls, *args, **kwds):
        print "A.__init__", args, kwds

class B(object):
    def __new__(cls, *args, **kwds):
        print "B.__new__", args, kwds
        return object.__new__(cls, *args, **kwds)
    def __init__(cls, *args, **kwds):
        print "B.__init__", args, kwds

Interactively A() then gives:

>>> A()
A.__new__ () {}
<__main__.B object at 0xb7bed0ec>

Yet [1] says: "[...] some rules for __new__: [...] If you return an
object of a different class, its __init__ method will be called."

Am I missing something? Is this documented somewhere else? Also it
would be nice if someone could point me to the function that implements
this in C. I didn't find anything in object.c or typeobject.c.

Best regards
Frank Benkstein.

[1] http://www.python.org/download/releases/2.2/descrintro/#__new__

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