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On Fri, Feb 13, 2015 at 12:35 PM, Guido van Rossum <guido@python.org> wrote:

IIUC you're proposing that the base class should \*try\* to construct an instance of the subclass by calling the type with an argument, and fail if it doesn't work. But that makes the whole thing brittle in the light of changes to the subclass constructor. Also, what should the argument be? The only answer I can think of is an instance of the base class.

No. The arguments should be whatever arguments are appropriate for the baseclass's \_\_init\_\_ or \_\_new\_\_. In the case of datetime.date that would be year, month, day.

Note that the original pure python prototype of the datetime module had date.\_\_add\_\_ and friends call self.\_\_class\_\_(year, month, day). Unfortunately, it looks like the original sandbox did not survive the the hg conversion, so I cannot provide a link to the relevant history.