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
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