On Tue, May 10, 2016 at 6:21 AM, Nick Coghlan <ncogh...@gmail.com> wrote:
> On 10 May 2016 at 02:30, Guido van Rossum <gu...@python.org> wrote: > > On Sun, May 8, 2016 at 7:52 PM, Nick Coghlan <ncogh...@gmail.com> wrote: > >> P.S. It occurs to me that a sufficiently sophisticated typechecker > >> might be able to look at all of the calls to "cls(*args, **kwds)" in > >> class methods and "type(self)(*args, **kwds)" in instance methods, and > >> use those to define a set of type constraints for the expected > >> constructor signatures in subclassses, even if the current code base > >> never actually invokes those code paths. > > > > Could you restate that as a concrete code example? (Examples of the > problems > > with "construction features" would also be helpful, probably -- abstract > > descriptions of problems often lead me astray.) > > Rectangle/Square is a classic example of the constructor signature > changing, so I'll try to use that to illustrate the point with a > "displaced_copy" alternate constructor: > > class Rectangle: > def __init__(self, top_left_point, width, height): > self.top_left_point = top_left_point > self.width = width > self.height = height > > @classmethod > def displaced_copy(cls, other_rectangle, offset): > """Create a new instance from an existing one""" > return cls(other.top_left_point + offset, other.width, > other.height) > (But why is it a class method? I guess the example could also use an instance method and it would still have the same properties relevant for this discussion.) > class Square: > def __init__(self, top_left_point, side_length): > super().__init__(top_left_point, side_length, side_length) > > At this point, a typechecker *could* have enough info to know that > "Square.displaced_copy(some_rectangle, offset)" is necessarily going > to fail, even if nothing in the application actually *calls* > Square.displaced_copy. > The question remains of course whether the type checker should flag Square to be an invalid subclass or merely as not implementing displaced_copy(). Anyway, at this point I believe we're just violently agreeing, so no need for another response. Though Serhiy may be unhappy with the lack of guidance he's received... -- --Guido van Rossum (python.org/~guido)
_______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
