On Fri, Feb 13, 2015 at 12:35 PM, Guido van Rossum <gu...@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.
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