return an object of a different class

Westley Martínez anikom15 at gmail.com
Thu Feb 17 15:39:48 CET 2011


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!




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