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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