__new__

[Alex Martelli]

Peter Otten <__peter__ at web.de> wrote:
   ...
> is as __new__ had left it). Thus, for a new-style class C, the statement
> x=C(23) is equivlent to the following code:
> 
>     x = C.__new__(C, 23)
>     if isinstance(x, C): C.__init__(x, 23)
   ...
> the "Nutshell" example should be changed to
> 
>     x = C.__new__(C, 23)
>     if isinstance(x, C): x.__init__(23)
> 
> to comply with the current implementation (I used Python 2.4).

Hmmm -- not quite, because in the new-style object model special methods
are taken from the type, NOT from the instance as the latter is saying.
E.g, if you change class B into:

class B(A):
    def __init__(self):
        print "init B"
        def f(): print 'from instance'
        self.__init__ = f

you'll see that while b.__init__() would print 'from instance',
instantiating A will not.  I think the right correction to the Nutshell
is therefore:

     x = C.__new__(C, 23)
     if isinstance(x, C): type(x).__init__(x, 23)

and this is how I plan to have it in the 2nd edition.


Alex