On 10 May 2016 at 02:30, Guido van Rossum <guido@python.org> wrote:
On Sun, May 8, 2016 at 7:52 PM, Nick Coghlan <ncoghlan@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) 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. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia