[Python-Dev] Return type of alternative constructors (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Tue May 10 09:21:57 EDT 2016

On 10 May 2016 at 02:30, Guido van Rossum <guido at python.org> wrote:

On Sun, May 8, 2016 at 7:52 PM, Nick Coghlan <ncoghlan at 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 at gmail.com | Brisbane, Australia

More information about the Python-Dev mailing list