[PyKDE] Re: Bug with QScrollView viewport binding

[Giovanni Bajo]

Phil Thompson <phil at riverbankcomputing.co.uk> wrote:

>> I understand the technical problem, but I believe it is a right
>> expectation that any call to QScrollView.viewport() return a
>> QWidget, as documented. The fact that thousands lines away I happen
>> to have a QObject reference to the same object should not
>> thoeretically make QScrollView.viewport() break its contract.
>
> Understood - but I'm concerned about possible unforeseen
> implications. Of the two obvious options...
>
> 1) Re-type the existing object when the more specific type is known.
> This is probably safe to do as far as the internals are concerned -
> but the idea of a Python object changing type under your feet sounds
> horrible.
>
> 2) Generate a new wrapper to the same C++ object with the more
> specific type. This has obvious problems with ownership (who calls
> the C++ dtor?), and things like "is" would fail. This is a real can
> of worms.

I don't like either as well.

The current logic, quoting you, is:
> The most down-cast type, yes - but *only* if it is a know type.
> Otherwise it resorts to the base type.

Maybe this could be adjusted to: "automatically cast-down to the most derived
type which is known". This way, calling parent() for QWidget's will always
create at least a QWidget, since all widgets' parents are widgets themselves.
Also, we would get a QWidget for the viewport too (which is derived from
QWidget, in fact), and this would solve the problem.

This is also what you say here, assuming I understood:

> rather than see if the type *is* a known type, see if the type
> is *derived* from a known type. Or maybe do the second only if the
> first fails (for speed and to ensure it doesn't break existing
> behaviour).

so looks like we agree on this solution.

>> Notice that there is *never* a need for downcast in PyQt, and this
>> would be a sole example.
>
> Some PyKDE uses have this requirement - hence sip.cast().

I would like to see some.

Giovanni Bajo