[Python-Dev] Odd new-style class __new__ behavior

[Michael Hudson]

Guido van Rossum <guido@python.org> writes:

> > On 30 Mar 2002, Michael Hudson wrote:
> > > Kevin Jacobs <jacobs@penguin.theopalgroup.com> writes:
> > > > Suppose I define:
> > > > 
> > > > class Foo(object):
> > > >   def __new__(cls):
> > > >     return 1
> > > > 
> > > > class Bar(object):
> > > >   def __new__(cls):
> > > >     return [1,2,3]
> > > > 
> > > > Python 2.2 returns:
> > > >   print Foo()
> > > >   > 1
> > > >   print Bar()
> > > >   > []
> > > > 
> > > > I would expect that Bar() should return [1,2,3].  Am I running into
> > > > some clever undocumented feature or a bug?  
> > > 
> > > Is tp_init being called on the returned list?
> 
> Correct.

Goody.  I always like it when my wild guesses turn out to be accurate :)

[...]
> Correct behavior is not to return a different type unless you know
> what its __init__ does.

>>> d = {1:1}
>>> d.__init__()
>>> d
{1: 1}
>>> l = [1]
>>> l.__init__()
>>> l
[]

This is at least a little odd, I'd have said.  Is this just a fluke of
implementation?

It's not like I really care, though.

Cheers,
M.

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