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

Guido van Rossum guido at python.org
Tue May 10 14:02:13 EDT 2016

On Tue, May 10, 2016 at 6:21 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

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 "displacedcopy" alternate constructor: class Rectangle: def init(self, topleftpoint, width, height): self.topleftpoint = topleftpoint self.width = width self.height = height @classmethod def displacedcopy(cls, otherrectangle, offset): """Create a new instance from an existing one""" return cls(other.topleftpoint + 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, topleftpoint, sidelength): super().init(topleftpoint, sidelength, sidelength)

At this point, a typechecker could have enough info to know that "Square.displacedcopy(somerectangle, offset)" is necessarily going to fail, even if nothing in the application actually calls Square.displacedcopy.

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...

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