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.
--
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