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

[Phil Thompson]

On 9 May 2018, at 8:04 am, J Barchan <jnbarchan at gmail.com> wrote:
> 
> ​
> 
> On 8 May 2018 at 22:02, Phil Thompson <phil at riverbankcomputing.com> wrote:
> 
> > On 8 May 2018, at 8:22 pm, J Barchan <jnbarchan at gmail.com> wrote:
> > 
> > ​
> > 
> > On 8 May 2018 at 18:14, Phil Thompson <phil at riverbankcomputing.com> wrote:
> > 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
> > 
> > ​Hi Phil,
> > 
> > Thanks for your reply.
> > 
> > I think one of us must be getting something wrong here.  I wonder if you're still expecting me to understand something which is obvious to you but not to me.
> > 
> > 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).
> > 
> > I'm not asking to have anything both ways.​  I'm just asking how to write code so that the overridden method behaves absolutely identically to the base method it's overriding.  Surely that must be possible?
> > 
> > I remind you: when I have no override for ​QSqlQueryModel.data() everything behaves perfectly.  I am saying: there is no problem, NULLs are handled as such and non-NULLs are correctly converted to their Python equivalent.  I do not know how NULLs work (what they are returned as), but everything just works.
> > 
> > As soon as I write an override which just returns the base method, it goes wrong on NULL.  If I put it the sip.autoconversion(False), it works on NULL but now returns a QVariant where it used to return a Python native type, I think.
> 
> ​​Correct - because data() returns a QVariant. With the auto-conversion the Python native type is automatically converted to a QVariant.
> 
> > All I want to know is: how can I write an override of  ​QSqlQueryModel.data() in Python/PyQt like:
> > 
> > def data(self, index: QtCore.QModelIndex, role=QtCore.Qt.DisplayRole) -> typing.Any:
> >      value = super().data(index, role)
> >      return value
> > 
> > such that it returns just exactly the same as QSqlQueryModel.data() would have done, please, please, please?
> 
> ​​You already have it - with the calls to autoenableconversion().
> 
> Phil
> 
> 
> ​​Correct - because data() returns a QVariant. With the auto-conversion the Python native type is automatically converted to a QVariant.​
> 
> ​Fine.  So are you saying I need to replicate the Python auto-conversion on the result I get back because I had to suppress the autoconversion?, and then return that?​  Is there a Pyton function, or what sort of code do I need to write, to achieve the same result as whatever the autoconversion would have done?  Because I have no idea...
> 
> ​​​You already have it - with the calls to autoenableconversion().
> 
> ​Phil, no I do not.  I very carefully typed in that I had changed over to precisely:
> 
> > > 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.

Unless there is a bug in re-enabling auto-conversion, that "other" code path is invoked by the call to the C++ implementation of data(), ie. while auto-conversion is disabled. So you get QVariants, so you can't have it both ways.

> That's exactly what you are saying ​"​You already have it - with the calls to autoenableconversion()." about, correct?  And I am saying: no, that code returns whatever correct value in the NULL case which I was originally complaining about, but now returns in other cases a QVariant back to my code where it used to return, say, a string.  Which then makes the calling code go wrong.  So with the calls to autoconversion it does not return the same result as if I had never written the override, which is precisely why I am stumped and asking the question....

You *have* to care about the code paths. To construct a QVariant from a Python value you call the QVariant constructor. To get the Python value from a QVariant you call its value() method.

Phil