[Python-Dev] Return type of alternative constructors

[Guido van Rossum]

On Sun, May 8, 2016 at 4:49 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 8 May 2016 at 08:39, Serhiy Storchaka <storchaka at gmail.com> wrote:
> > Should alternative constructor call __new__ and __init__ methods? Thay
> can
> > change signature in derived class.
>
> I think this is typically the way to go (although, depending on the
> specific type, the unpickling related methods may be a more
> appropriate way for the alternate constructor to populate the instance
> state)
>

Honestly, either of these sounds like fragile, even though I really want
the alternative constructor to return an instance of the subclass (else why
invoke it through the subclass).


> > Should it complain if __new__ or __init__ were overridden?
>
> If there are alternate constructors that depend on either the
> signature of __new__/__init__, unpickling support, or some other
> mechanism for creating new instances, this should be mentioned in the
> class documentation as a constraint on subclasses - if subclasses
> don't want to meet the constraint, they'll need to override the
> affected alternate constructors.
>

Putting this constraint in the docs sounds fragile too. :-(


> Cheers,
> Nick.
>
> P.S. The potential complexity of that is one of the reasons the design
> philosophy of "prefer composition to inheritance" has emerged -
> subclassing is a powerful tool, but it does mean you often end up
> needing to care about more interactions between the subclass and the
> base class than you really wanted to.
>

Indeed!

We could also consider this a general weakness of the "alternative
constructors are class methods" pattern. If instead these alternative
constructors were folded into the main constructor (e.g. via special
keyword args) it would be altogether clearer what a subclass should do.

-- 
--Guido van Rossum (python.org/~guido)
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