How to link custom widgets to model data

superaga

Hi experts!

I'd like to make a custom dialog which uses custom widgets (I made) to show the values from a QAbstractTableModel.

From what I understood (in this guide) here what I need:

An instance of a QAbstractTableModel.
A "view" made using widgets (including my customs).
An instance of the QDataWidgetMapper used to "link" the model data to "these" widgets.

I read also that the widgets need to have the user property which is used to transfer the data back and forth to and from the model.

And.. I read again that: "It is possible to set an item delegate to support custom widgets. By default, a QItemDelegate is used to synchronize the model with the widgets."

Which adds another "character" to the whole picture...

Example:
Let's say that I want to show changing values of my model (a small table (2, 2)) where the first column contains floats and the other boolean values. The first 2 widgets are just a QLabel, while the others are my custom widget (LEDs).

What should I need from the above "classes" to create this app?
How do I add the user property to my custom (widget) class?
- Sorry but I didn't find an example for that.
What can QItemDelegate do for me?
- It seems more something related to fine tuning, isn't it?

As usual many thanks for your answers!

Kind regards,
AGA

superaga

Hi @JonB,

@JonB said in How to link custom widgets to model data:

That would suggest QDataWidgetMapper is only interested in providing editing facilities and therefore utilizing an "editing" control. So it's not set up [by default] for working with a QLabel.

That's makes a lot of sense now...
... And yes, using the addMapping method as from your linked page worked as expected...

Here the code that solved also this point:

...
...
self.mapper.addMapping(self.lbl_val_1, 1, b'text')
self.mapper.addMapping(self.lbl_val_2, 2)
...
...

I also updated as @SGaist told me my custom Led class (I'm using PyQt not PySide) as:

...
...
@pyqtProperty(str)
def value(self):
    return self._value

@value.setter
def value(self, value):
    self._value = value
    
    if self._value == 1:
        self.on()
    else:
        self.off()
...
...

But didn't work. I spend basically yesterday afternoon trying to understand what could cause the issue and I think that despite I made a mistake, I think that I found a "bug".

To make the long story short the type of the Q_PROPERTY in my decorator and the data type were different.

At the begin I (wrongly) casted my returned data method (within the model) to be str:

return str(self._data[index.row()][index.column()])

Then I removed the str() cast, so data were back to their original float form, but my decorator was still set to receive string: @pyqtProperty(str).

Running this never returned any runtime error or warning, but didn't work.
Of course, since we are talking about Python (which is also dynamically typed) I'm not sure if define this behavior wrong (here the double quotes around the bug word).

Changing the decorator type to int solved the issue.

I really wanted to attach my working code in order to provide help to anyone, but looks that I can't (Error: You don't have enough privileges for this action).

Please let me know how to fix this.
Thanks!

Kind reagrds,
AGA

SGaist

Hi,

The delegates are used in conjunction with the views which you are not using so no need to worry about them.

For the user property, see the Property class. You essentially have to set the user parameter to True.

superaga

Hi @SGaist,

thanks for your answer! I updated my widget class accordingly.
... But I discovered that I have at least another issue first.

Here where I am:

I made a simple Model/View example that plots in a QTableView the data:

I created a custom widget LED:

I then redesigned my view in order to "remap" the table cells to my widgets.
To start easy I just added few QLabel widgets.

Here my view.py

class MyView(QWidget):
    def __init__(
        self,
        model_table: QAbstractTableModel,
        parent: QObject = None,
    ) -> None:
        super().__init__(parent=parent)

        # create layout
        self.initUI(model_table=model_table)

    def initUI(self, model_table: QAbstractTableModel) -> None:
        # make the GUI
        vbox = QVBoxLayout(self)
        # make widgets
        self.form_layout = QFormLayout()
        self.lbl_txt_1 = QLabel("Cell 1 value:")
        self.lbl_txt_2 = QLabel("Cell 2 value:")
        self.lbl_txt_3 = QLabel("Cell 3 value:")
        self.lbl_val_1 = QLabel()
        self.lbl_val_2 = QLabel()
        self.lbl_val_3 = QLabel()
        # add widgets to layout
        self.form_layout.setWidget(0, QFormLayout.ItemRole.LabelRole, self.lbl_txt_1)
        self.form_layout.setWidget(0, QFormLayout.ItemRole.FieldRole, self.lbl_val_1)
        self.form_layout.setWidget(1, QFormLayout.ItemRole.LabelRole, self.lbl_txt_2)
        self.form_layout.setWidget(1, QFormLayout.ItemRole.FieldRole, self.lbl_val_2)
        self.form_layout.setWidget(2, QFormLayout.ItemRole.LabelRole, self.lbl_txt_3)
        self.form_layout.setWidget(2, QFormLayout.ItemRole.FieldRole, self.lbl_val_3)
        vbox.addLayout(self.form_layout)
        # add the mapper
        self.mapper = QDataWidgetMapper()
        self.mapper.setModel(model_table)
        self.mapper.addMapping(self.lbl_val_1, 1)
        self.mapper.addMapping(self.lbl_val_2, 2)
        self.mapper.addMapping(self.lbl_val_3, 3)
        self.mapper.toFirst()

But I've got just this "empty" dialog which doesn't show my data values:

I then tried to use both the QLineEdit and QSpinBox as in the documentation example, but I still have the very same behavior:

What I'm still missing?
One more: How my mapped value is passed to the widget?
- I think through the Q_PROPERTY, correct?
  - I'm wondering then: ... what if I want/need to cast (string/float) my data before it would arrive at the widget?
  - It might be this the reason why my label were empties?, But what about then the QSpinBox which uses instead int rather than string?

So I still have these open questions before move to use my custom widget...

Many thanks!

superaga

Hi experts!

anyone that can try to help me to understand what I'm missing?

Many thanks!

JonB

@superaga
The behaviour you show --- empty/defualt value mapped widgets --- would be the case if there is no data in your model. Your code shows no evidence there is any data, or suitable data in the specified columns.

superaga

Hi @JonB,

The model contains the same (random generated) data that I showed in the first message.
I just updated the view to contain both the current QFormLayout and the QTableView of the "standard version", to provide the evidence that the Model/View architecture is working as expected.

The updated view code basically adds the table which is linked to the same model as well.

...
...
self.form_layout.setWidget(2, QFormLayout.ItemRole.LabelRole, self.lbl_txt_3)
self.form_layout.setWidget(2, QFormLayout.ItemRole.FieldRole, self.lbl_val_3)
v_splitter = QSplitter(QtCore.Qt.Orientation.Vertical)
self.widget = QWidget()
self.widget.setLayout(self.form_layout)
v_splitter.addWidget(self.widget)
# insert table
self.table_view = QTableView()
v_splitter.addWidget(self.table_view)
vbox.addWidget(v_splitter)
# add the mapper
self.mapper = QDataWidgetMapper()
# connect dictionaries to views
self.mapper.setModel(model_table)
self.table_view.setModel(model_table)
self.mapper.addMapping(self.lbl_val_1, 1)
...
...

Where could be the error?

I still have no clear how can I manipulated the data type of the model data before passing it to the widget.

Many thanks!

JonB

@superaga
I don't know, I never had any trouble getting a QDataWidgetMapper to work. Could you start by:

Stop all the values changing. Get it working first with values in the table which do not change all the time; and
Use the widget which is suitable for your values. Since they look like all floating point QSpinBox is not right, use a QDoubleSpinBox.

superaga

Hi @JonB,

I update the model and the view accordingly, but it is still not working.

Now model just contains these values:

...
self._data = [[0,1,2,3], [3,2,1,0]]
self._rows = len(self._data)
self._columns = len(self._data[0])
...
...
# reimplement the model methods
def rowCount(self, parent: QModelIndex) -> int:
    if parent.isValid():
        return 0
    return self._rows

def columnCount(self, parent: QModelIndex) -> int:
    if parent.isValid():
        return 0
    return self._columns

def data(self, index: QModelIndex, role: Qt.ItemDataRole) -> Any:
    if role != Qt.ItemDataRole.DisplayRole:
        return QtCore.QVariant()
    # in all other cases return data
    return str(self._data[index.row()][index.column()])
...

Data are now just integers, so using QSpinBox or QDoubleSpinBox didn't make any difference.

Are there other tests that I can perform?

Thanks!

SGaist

@superaga your model returns nothing except for DisplayRole (that comment is highly misleading). QDataWidgetMapper uses Qt.EditRole to get the value in the editor.

JonB

@superaga
@SGaist is of course correct now that you show your model code. That's why we needed to see that from the start! :)

superaga

Hi @JonB and @SGaist,

😱
That's a very bad mistake! I'm so sorry... 😔

I just fixed the model data method to be:

def data(self, index: QModelIndex, role: Qt.ItemDataRole) -> Any:
    if not index.isValid():
        return None
    if role == Qt.ItemDataRole.DisplayRole:
        return str(self._data[index.row()][index.column()])

Now it is correct... But it is still not working (I still have the very same picture of my previous message).

Why the QTableView shows the values instead?

I read again the ItemDataRole description and looks good now.

I tried to understand where the issue is but I couldn't find any wrong.

JonB

@superaga
I don't see how what you have is right or that you have heeded what @SGaist said. Your method still returns nothing for the edit role? The usual code would be:

    if role == Qt.ItemDataRole.DisplayRole || role == Qt.ItemDataRole.EditRole:
        return str(self._data[index.row()][index.column()])

It's also not good to return str(...) from data() when the underlying data has a perfectly reasonable type of its own, such as a number. Forcing to str will stop all kinds of things working correctly, e.g. sorting. It may also stop a QDoubleSpinBox working.

superaga

Hi @JonB,

Ok, now it works.

I understood that: I didn't have a clear picture about the ItemDataRole since I "deliberately overlooked" the EditRole.
I was convinced that to show read only data the DisplayRole would have been enough, but I was wrong. Could you please suggest me a link where this is well explained?
I thought that EditRole was necessary only to let the data be editable.

Regarding the data casting on the data method return I completely agree: this is completely wrong and must not be done.

One more (if I can) could you also explain me (or point me to) how could I cast data (i.e. from float to str)? I still miss this "hidden" step between the model data return and the mapper fetch.

Super thanks!

JonB

@superaga
QDataWidgetMapper expects to allow editing I think, and doesn't particularly know/care that your model might be read-only. If @SGaist says it always uses EditRole then that's what it does.

I have never used a QDataWidgetMapper with a QLabel, so not yet sure where the str() needs doing. If you temporarily go back to data() doing the str() (for the edit case, and for the column for Cell 1 value at least) does that then show the value on the label? I would test that first, else it's maybe a QLabel rather than a str issue?

superaga

Hi @JonB, and @SGaist,

I somehow missed that QDataWidgetMapper needs the EditRole, I apology.

I modified the data method for the model as you described but the QLabel is still empty. I assume than that, for some reasons, the QLabel is not suited to be used within the QDataWidgetMapper...
I'll try now to progress in the view to replace the standard widget with mine.

Many thanks to both of you

JonB

@superaga said in How to link custom widgets to model data:

I modified the data method for the model as you described but the QLabel is still empty. I assume than that, for some reasons, the QLabel is not suited to be used within the QDataWidgetMapper...

That would suggest QDataWidgetMapper is only interested in providing editing facilities and therefore utilizing an "editing" control. So it's not set up [by default] for working with a QLabel. Doubtless it can be done....

Ah, yes, now this is all to do with that "user" property you were asking about :) A straightforward example showing how easy it is to change for you is QDataWidgetMapper not working with QLabels. You just need to change your addMapping() lines to write their text correctly. And after that you might understand how the property stuff works, if it's relevant to your widget....

SGaist

Since python does not enforce types, I would wrap all returned values in QVariant since that's the type used in C++ and especially when setting properties like QDataWidgetMapper.

superaga

Hi @JonB,

@JonB said in How to link custom widgets to model data:

That would suggest QDataWidgetMapper is only interested in providing editing facilities and therefore utilizing an "editing" control. So it's not set up [by default] for working with a QLabel.

That's makes a lot of sense now...
... And yes, using the addMapping method as from your linked page worked as expected...

Here the code that solved also this point:

...
...
self.mapper.addMapping(self.lbl_val_1, 1, b'text')
self.mapper.addMapping(self.lbl_val_2, 2)
...
...

I also updated as @SGaist told me my custom Led class (I'm using PyQt not PySide) as:

...
...
@pyqtProperty(str)
def value(self):
    return self._value

@value.setter
def value(self, value):
    self._value = value
    
    if self._value == 1:
        self.on()
    else:
        self.off()
...
...

But didn't work. I spend basically yesterday afternoon trying to understand what could cause the issue and I think that despite I made a mistake, I think that I found a "bug".

To make the long story short the type of the Q_PROPERTY in my decorator and the data type were different.

At the begin I (wrongly) casted my returned data method (within the model) to be str:

return str(self._data[index.row()][index.column()])

Then I removed the str() cast, so data were back to their original float form, but my decorator was still set to receive string: @pyqtProperty(str).

Running this never returned any runtime error or warning, but didn't work.
Of course, since we are talking about Python (which is also dynamically typed) I'm not sure if define this behavior wrong (here the double quotes around the bug word).

Changing the decorator type to int solved the issue.

I really wanted to attach my working code in order to provide help to anyone, but looks that I can't (Error: You don't have enough privileges for this action).

Please let me know how to fix this.
Thanks!

Kind reagrds,
AGA

superaga

It is not possible attach files, so here the code (from an external hosting):
https://mega.nz/file/FDIThArB#wf8nzrveUSlaj76gx_Qmtu7yv_1K4Ivj_BRfkdIB6mw

I hope it helps!
AGA