[PyQt] PyQt 5.7, QSqlQueryModel.data() sub-classed override bug?

[Phil Thompson]

On 8 May 2018, at 9:04 am, J Barchan <jnbarchan at gmail.com> wrote:
> 
> ​Now I'm finding that, with the fix discussed, while my overridden function definition correctly handles database NULLs, it "goes wrong" (as in, different behaviour from before) in certain other cases, returning a QVariant where it did not do so before (it returned the converted, native Python type).​
> 
> 1. So long as I do not override QSqlQueryModel.data() at all, there is absolutely no problem --- both database NULL and auto-conversion of non-NULL to Python native type work fine, and are distinct.  This is the situation I need.
> 
> 2. I need to override QSqlQueryModel.data() for my own purposes.  If I write just:
> def data(self, index: QtCore.QModelIndex, role=QtCore.Qt.DisplayRole) -> typing.Any:
>     value = super().data(index, role)
>     return value
> Some data conversion happens, such that I no longer get NULL back where the value is NULL --- instead it is converted to '' if string or 0 if int.  This was my original problem and is not acceptable.
> 
> 3. Following our discussion, I change that to:
> def data(self, index: QtCore.QModelIndex, role=QtCore.Qt.DisplayRole) -> typing.Any:
>     was_enabled = sip.enableautoconversion(QtCore.QVariant, False)
>     value = super().data(index, role)
>     sip.enableautoconversion(QtCore.QVariant, was_enabled)
>     return value
> Now I correctly get whatever for database NULL, which works.  However, some other path of code, on some quite different non-NULL value, gets back a QVariant where it used to get a string.  I don't know what that path of code is, but I don't think I should care.
> 
> So, what I need is: code which allows me to override QSqlQueryModel.data() but returns the original data() value unchanged, just like it used when I did not put any override in (case #1).  It must do whatever to correctly deal with NULL & non-NULL, just like the non-overridden QSqlQueryModel.data() does.
> 
> (In PyQt 5.7) What exact code can I put into the override to achieve just that, please?  Surely that can be done, no?

You can't have it both ways. Either you let PyQt automatically convert to/from QVariant (and you lose the detection of nulls) or you do it yourself (converting to Python using the value() method).

By the way, I've just noticed a bug in the docs which says (incorrectly) that null QVariants are converted to None and vice versa.

Phil