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

Nick Coghlan ncoghlan at gmail.com
Sun May 8 07:49:20 EDT 2016

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)

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.

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.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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