On Sat, Feb 14, 2015 at 7:23 AM, Steven D'Aprano <st...@pearwood.info>
wrote:
> Why can't int, str, list, tuple etc. be more like datetime?
They are. In all these types, class methods call subclass constructors but
instance methods don't.
>>> class Int(int):
... pass
...
>>> Int.from_bytes(bytes([1,2,3]), 'big')
66051
>>> type(_)
<class '__main__.Int'>
>>> Int(1) + 1
2
>>> type(_)
<class 'int'>
In the case of int, there is a good reason for this behavior - bool. In
python, we want True + True == 2. In numpy, where binary operations
preserve subclasses, you have
>>> import numpy
>>> numpy.bool_(1) + numpy.bool_(1)
True
I don't see a similar argument for the date class, however. Given
date.{to|from}ordinal(), date subclasses are pretty much bound to have
timedelta addition satisfy (d + td).toordinal() == d.toordinal() +
td.days. Any other definition would be fighting the baseclass design and
would be better implemented via containment.
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