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

Thu May 3 14:50:26 BST 2018

Hang on.  This is an area I do not understand, doubtless you do.

The C++ QSqlQueryModel::data() method is supposed to return a QVariant.  Am
I maybe losing the QVariant-ness when I write my own PyQt overload which
returns the base method's result, because Python-esque conversion is going
on?

I'm trying to understand
http://pyqt.sourceforge.net/Docs/PyQt5/pyqt_qvariant.html, but I don't
really.  I do note:
> There is no obvious way to represent a null QVariant
<http://pyqt.sourceforge.net/Docs/PyQt5/api/QtCore/qvariant.html#PyQt5-QtCore-QVariant>
as a standard Python object.

Is there a connection between this and the fact that it goes wrong when the
value it should be returning is the NULL returned from the SQL query?

On 3 May 2018 at 13:55, J Barchan <jnbarchan at gmail.com> wrote:

>
>
> On 3 May 2018 at 13:05, Phil Thompson <phil at riverbankcomputing.com> wrote:
>
>> On 3 May 2018, at 12:25 pm, J Barchan <jnbarchan at gmail.com> wrote:
>> >
>> > 
>> > I am finding (in PyQt 5.7 at least) that sub-classing QSqlQueryModel
>> and overriding its data() method produces an incorrect result when the
>> value retrieved from a MySQL database is NULL.
>> >
>> > Full details are in https://forum.qt.io/topic/9036
>> 3/inexplicable-qsqlquerymodel-handling-of-null-value, and particularly
>> post # https://forum.qt.io/topic/90363/inexplicable-qsqlquerymodel-
>> handling-of-null-value.  Nobody has tried it in C++ for me to date to
>> verify, but I'm suspecting this might be a PyQt bug?
>> >
>> > Briefly:
>> > My SELECT query returns a column which is NULLable, and has NULL as its
>> value.  Where I expect "blank" as the end value, I actually get, for
>> example, 0 if the column type is int or '' if the type is string, etc.
>> >
>> > This is when I sub-class QSqlQueryModel.  If all I have is:
>> > class DBQueryModel(QtSql.QSqlQueryModel):
>> >     def __init__(self, parent=None):
>> >         super().__init__(parent)
>> > I get the "NULL"/"blank".  However, as soon as I add just:
>> > def data(self, index: QtCore.QModelIndex, role=QtCore.Qt.DisplayRole)
>> -> typing.Any:
>> >     return super().data(index, role)
>> > I get those values instead of NULL.
>> >
>> > Note that my override is based on the Qt definition of the method at
>> http://doc.qt.io/qt-5/qsqlquerymodel.html#data:
>> > QVariant QSqlQueryModel::data(const QModelIndex &item, int role =
>> Qt::DisplayRole) const
>> >
>> > Note that the default for role is Qt::DisplayRole.  However, in
>> QtSql.py I see:
>> > def data(self, QModelIndex, role=None): # real signature unknown;
>> restored from __doc__
>> >     """ data(self, QModelIndex, role: int = Qt.DisplayRole) -> Any """
>> >     pass
>> >  You will notice that the comment shows the default should be
>> Qt.DisplayRole, but the declaration defaults it to None instead.
>>
>> What is QtSql.py?
>>
>> If you want to know the signature of a method pss it to help().
>>
>> > I don't know enough to be sure, but would that be the underlying cause
>> of the unexpected behaviour?
>> >
>> > FWIW, I have tried making my override be:
>> > def data(self, index: QtCore.QModelIndex, role=None)
>> > instead, but same bad behaviour.
>> >
>> > 1. Is this indeed a bug in PyQt, and the cause of my issue?
>>
>> No and no.
>>
>> > 2. If so, I presume you (Phil!) will be kind enough to fix.  However,
>> for my part I am stuck with PyQt 5.7 for the foreseeable future.  If the
>> fix is indeed to change code in the latest/next release, is there anything
>> I can do in existing code (my override) to make it work in 5.7, as a
>> workaround?  (in real code I need the override, as I do other processing)
>> >
>> > My coding has come to halt as I cannot proceed without a fix.  So I
>> should be obliged for any early response as to whether this is the cause of
>> my woes.  I do realise PyQt support/fixes are quite voluntary, and so thank
>> whoever in advance!
>>
>> Phil
>
>
> For QtSql.py:
>
> Hmm, I had not realised.  I use PyCharm as my IDE.  From there, while I
> am coding, I can click on anything PyQt and ask for "Go to
> definition/declaration".  The editor then 9in this case) opens me up into a
> file named QtSql.py, showing me in this case [extract]:
>
> class QSqlQueryModel(__PyQt5_QtCore.QAbstractTableModel):
>>
>> ...
>>
>> def columnCount(self, parent=None, *args, **kwargs): # real signature unknown; NOTE: unreliably restored from __doc__
>>     """ columnCount(self, parent: QModelIndex = QModelIndex()) -> int """
>>     pass
>>
>> def data(self, QModelIndex, role=None): # real signature unknown; restored from __doc__
>>     """ data(self, QModelIndex, role: int = Qt.DisplayRole) -> Any """
>>     pass
>>
>> def endInsertColumns(self): # real signature unknown; restored from __doc__
>>     """ endInsertColumns(self) """
>>     pass
>>
>>
> etc.  I had *assumed* this was a file supplied with PyQt.  I guess now
> it's "generated on the fly" by PyCharm (I see its path is in a PyCharm
> temporary directory).  At other times, it might open, say,
> /usr/lib/python3/dist-packages/PyQt5/QtCore.pyi, which I think is a file
> you supply.  Oh, at the head of this QtSql.py I see:
>
> # encoding: utf-8
> # module PyQt5.QtSql
> # from /usr/lib/python3/dist-packages/PyQt5/QtSql.cpython-35m-x86_64-linux-gnu.so
> # by generator 1.145
> # no doc
>
>
> You'll probably understand all this better than I!
>
>
>
> For my problem:
>
> I think I now understand better why it's not a PyQt method definition
> issue.
>
> However, from the linked Qt forum discussion, I'm stuck between a rock & a
> hard place, because the only help I'm getting is that it might be a PyQt
> issue.  I do not have C++ to try that out.  So, I wonder if I might ask you
> if you can make any suggestion as to the cause, even if it is not a PyQt
> issue, given that you are familiar with Qt at least?
>
> To summarise my problem as briefly as possible:
>
> 1.
> I start with:
>
> model = QtSql.QSqlQueryModel(self)
> model.setQuery("SELECT LandlordNo, SMTPAccountId FROM landlords WHERE SMTPAccountId IS NULL")
>
> # or plain "SELECT NULL AS SMTPAccountId", to eliminate anything about the column definition being an issue
>
> rowCount = model.rowCount()
> if rowCount > 0:
>     rec = model.record(0)
>     field = rec.field("SMTPAccountId")
>     isn = field.isNull()
>
> SMTPAccointId returns NULL from MySQL.  *At this point field.isNull()
> correctly returns True.*
>
> 2.
> I sub-class QSqlQueryModel, and use that, with quite simply, exactly:
>
> class DBQueryModel(QtSql.QSqlQueryModel):
>     def __init__(self, parent=None):
>         super().__init__(parent)
>
> and use that sub-class in place of QSqlQueryModel:   model = DBQueryModel(
> self)
> *And it this point point field.isNull() *still* correctly returns True.*
>
> 3.
> Then I add *just exactly this* to my sub-class:
>
> def data(self, index: QtCore.QModelIndex, role=QtCore.Qt.DisplayRole) -> typing.Any:
>     return super().data(index, role)
>
> (I've also tried role=None) You can see that simply calls the base class
> method.  *But now field.isNull() returns False!!*  The application sees 0
> instead of NULL for the value (SMTPAccountId is declared INT NULL), or ''
> if I use SELECT NULL AS SMTPAccountId so it counts as string.
>
>
> This leaves me completely stumped.  I have no idea where the problem is
> (there shouldn't be a problem!).  I have to sub-class the data() method
> for other purposes, but then it handles NULL (only) incorrectly.
>
> Would you have any idea what is going on here?  My thanks in advance.
>
>
> --
> Kindest,
> Jonathan
>

-- 
Kindest,
Jonathan
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