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

[J Barchan]

On 9 May 2018 at 08:59, Phil Thompson <phil at riverbankcomputing.com> wrote:

> 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


​Dear Phil,

Just so you know, I am not sitting idly, I am doing my best to debug and
find out what's going on.

The bit I don't get, and I'm not sure you are 100% appreciating, is that
​if I do *not* define any override the base method *returns whatever
correct result in all cases.* Whether the data value is database NULL or
non-NULL, the calling code gets the right answer in all situations.  *If I
did not need to override the method I would not have any problem.*  Now,
with no override I cannot place a breakpoint to examine what the base
method actually returns when, so I cannot discover what it is returning, I
can only say everything behaves 100% correctly.
*I don't know how it gets it right, but somehow it does.*

*As soon as I do introduce the override*, I can play with adding in
autoconversion or not, I can call a QVariant.value() or not, I can do what
ever I can think of.  But it *never* returns whatever it used to without
the override such that the calling code gets both the NULL and the non-NULL
cases correct.  It *always* then gets one or the other case wrong.

For the record, when I ask in a Qt forum the C++ people say "we have tried
overriding the method to just return the base method result and there is
absolutely no problem, all is well, just as if we had never overridden the
method, so we don't know what your problem is, it must be something in
Python/PyQt".

It must (ought to, surely?) be possible to get the PyQt override to be able
to just return/behave as if I had not overridden an override, and
effectively behave just as if I had never written an override and just let
it perform the base class call as it would have done, but *nothing* I have
tried seems to behave like that..... ?


-- 
Kindest,
Jonathan
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