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

J Barchan jnbarchan at gmail.com
Thu May 3 14:54:52 BST 2018 

Ah ha!  I read in
https://www.riverbankcomputing.com/pipermail/pyqt/2016-April/037326.html:

>>>>
* I am currently updating QGIS to PyQt5 (and Qt5 and Python3)>>>>
Since this update, NULL QVariant strings are converted to empty
strings '' and all numbers to 0 when converted from C++ to python
objects.*

And that is exactly what I am experiencing: the string-NULL is giving me
empty string and the int-NULL is giving me 0.  So isn't this a PyQt issue I
am seeing after all??

On 3 May 2018 at 14:50, J Barchan <jnbarchan at gmail.com> wrote:

> 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
>



-- 
Kindest,
Jonathan
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