__new__ - comp.lang.python

Message from discussion __new__

Alex Martelli

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More options Nov 6 2005, 11:12 am

Newsgroups: comp.lang.python

From: al...@mail.comcast.net (Alex Martelli)

Date: Sun, 6 Nov 2005 08:12:19 -0800

Local: Sun, Nov 6 2005 11:12 am

Subject: Re: __new__

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Peter Otten <__pete...@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

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