return an object of a different class

[Westley Martínez]

On Thu, 2011-02-17 at 15:56 +0000, MRAB wrote:
> On 17/02/2011 14:39, Westley Martínez wrote:
> > On Thu, 2011-02-17 at 11:43 +0000, Steven D'Aprano wrote:
> >> On Thu, 17 Feb 2011 12:02:28 +0100, Jean-Michel Pichavant wrote:
> >>
> >>> Karim wrote:
> >>>> [snip]
> >>>>> If you don't want to use a factory function I believe you can do this:
> >>>>>
> >>>>> class MyNumber(object):
> >>>>>       def __new__(cls, n):
> >>>>>           if n<= 100:
> >>>>>               cls = SmallNumbers
> >>>>>           else:
> >>>>>               cls = BigNumbers
> >>>>>           return object.__new__(cls, n)
> >>>>>       ...
> >>>>>
> >>>>> Chard.
> >>>>
> >>>> Very beautiful code great alternative to factory method! To memorize
> >>>> this pythonic way.
> >>>>
> >>>> Regards
> >>>> Karim
> >>> Do you think  that the MyNumber constructor returning something else
> >>> than a MyNumber instance is the pythonic way ? It would rather be the
> >>> cryptonic way ! (haha)
> >>
> >>
> >> Support for constructors returning something other than an instance of
> >> the class is not an accident, it is a deliberate, and useful, design. The
> >> Fine Manual says:
> >>
> >>      object.__new__(cls[, ...])
> >>
> >>      Called to create a new instance of class cls. [...]
> >>      The return value of __new__() should be the new object
> >>      instance (usually an instance of cls).
> >>      [...]
> >>      If __new__() does not return an instance of cls, then
> >>      the new instance’s __init__() method will not be invoked.
> >>
> >>
> >> http://docs.python.org/reference/datamodel.html#basic-customization
> >>
> >> So while it is *usual* for the constructor to return an instance of the
> >> class, it's not compulsory, and returning other types is explicitly
> >> supported.
> >>
> >> To answer your question about whether this is Pythonic... here's a small
> >> clue from Python 2.5:
> >>
> >>>>> n = int("4294967296")  # 2**32
> >>>>> type(n)
> >> <type 'long'>
> >>
> >>
> >>
> >> So, yes, absolutely, it is not only allowed for class constructors to
> >> return an instance of a different class, but there is precedence in the
> >> built-ins.
> >>
> >>
> >>
> >> --
> >> Steven
> > Python 3 removed longs because they were ... cryptonic!
> >
> Strictly speaking, they weren't removed. ints were removed and long was
> renamed int.
My point stands.