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On Sat, Feb 14, 2015 at 7:23 AM, Steven D'Aprano <steve@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(\_)

>>> Int(1) + 1

>>> type(\_)

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.