PyQt 5: findChild returns None

Hubris

Hi all,

I have created a tabbed window where the labels and qlineedits are created using a for loop, distinguishing between the objects by having different objectName.

class TabWidget(QWidget):
    def __init__(self, parent):
        super(QWidget, self).__init__(parent)

item_names = [
            "so",
            "dc",
            "cn",
            "co",
            "ea",
            "tn",
            "sz",
            "m1",
            "m2",
            "ad",
            "de",
            "dl",
            "sd",
            "cp",
            "sh",
        ]

for itemnum in range(0, len(item_names) - 2):
            self.label = QLabel(form_labels[itemnum])
            self.qledit = QLineEdit()
            if item_names[itemnum] == "so" or item_names[itemnum] == "dc":
                self.qledit.setEnabled(False)
            self.label.setObjectName(f"{item_names[itemnum]}_l")
            self.qledit.setObjectName(f"{item_names[itemnum]}_qle")
            self.qledit.setFixedWidth(500)
            form_layout.addRow(self.label, self.qledit)

This TabWidget class is then called in the setupUI function of the QMainWindow:

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        print("Initialising UI...")
        self.tab_widget = TabWidget(self)
        print("UI initialised.")
        self.setCentralWidget(self.tab_widget)
        self.move(300, 150)
        self.setWindowTitle("TopperWorks DE Utility")
        self.show()

I have omitted the layout formatting of the TabWidget as I believe that to be irrelevant to the question.
I want to change the value of one of the qlineedits that I have created from the for loop with findChild:

qle_by_name = self.findChild(QLineEdit, "dc_qle")
qle_by_name.setText("Test")

However, findChild returns None. The findChild usage is within the TabWidget class.

Thanks.

JonB

@Hubris
OK, let's go back to basics :)

You add your widgets via form_layout.addRow(self.label, self.qledit). Can you please

Show what form_layout is
Show how it is connected to your widget hierarchy, what its parent is

We should like to try print(form_layout.findChildren()) to verify that can find the widgets you added. If form_layout is a layout (e.g. QFormLayout) then this needs to be called on its parent widget, e.g. form_layout.parentWidget().findChildren().

Furthermore it is not clear when you are trying these findChild/findChildren() calls. They will only work when the layout/sub-widgets you have added have actually been added onto their parent to find from. For example, if you have added them to form_layout but that has not yet been set as the layout on a widget you won't find them.

jsulm

@Hubris You could iterate over all children (https://doc.qt.io/qt-6/qobject.html#children) and print out their names to see whether you really have such an object.
As far as I can see you also never set the parent on the labels and line edits.

JonB

@Hubris
In addition/follow up to @jsulm

form_layout.addRow(self.label, self.qledit)

As code stands form_layout is a non-existent local variable.

Separate issue:

            self.label = QLabel(form_labels[itemnum])
            self.qledit = QLineEdit()

You are executing these statements multiple times inside a loop. These should not be assigned to (the same) class member variables.

Hubris

@jsulm Thanks for the reply. I used self.findChildren to return the QLineEdit, QPushbutton, and QLabel children but it just returns an empty array.

I also tried to set the parent for the objects by using self.qledit.setParent(TabWidget), however I encounter the following error:

setParent(self, QWidget): argument 1 has unexpected type 'sip.wrappertype'

Hubris

@JonB Thanks for the reply.

form_layout is declared as a QFormLayout object earlier in the code. I just omitted that section. If you'd need that information I am happy to provide the whole code.

As for assigning to the same class member variables inside the for loop,

I am making a form which has a lot of multiple repeating QLabels, and QLineEdits. If I do not instantiate a new label for every loop, I would just be modifying the same QLabel/QLineEdit every iteration, and the result of that is undesirable.

Is this idea not the right approach?

Here is the output of my code:

Each QLabel and QLineEdit has a distinct objectName.

JonB

@Hubris
That's fine.

The observation about your class variables is probably not relevant. They end up referencing just the final label/edit you happen to create from the loop. I doubt that is useful to you as a member variable reference.

            label = QLabel(form_labels[itemnum])
            qledit = QLineEdit()
            ...

Wouldn't this suffice?

If I wanted to debug your situation I'd start with @jsulm's

@Hubris You could iterate over all children (https://doc.qt.io/qt-6/qobject.html#children) and print out their names to see whether you really have such an object.

You could also do a quick self.findChild(QLineEdit) to see whether it finds any line edits.

Hubris

@JonB Once again thank you for the reply.

I have done your suggestion and that indeed did suffice.

I have considered @jsulm's reply. I used the following functions:

...
        print(self.findChildren(QWidget))
        print(self.findChildren(QLabel))
        print(self.findChildren(QLineEdit))
        print(self.findChildren(QPushButton))
        print(self.findChild(QLineEdit))
...

which returned:

[]
[]
[]
[]
None

JonB

@Hubris
OK, let's go back to basics :)

You add your widgets via form_layout.addRow(self.label, self.qledit). Can you please

Show what form_layout is
Show how it is connected to your widget hierarchy, what its parent is

We should like to try print(form_layout.findChildren()) to verify that can find the widgets you added. If form_layout is a layout (e.g. QFormLayout) then this needs to be called on its parent widget, e.g. form_layout.parentWidget().findChildren().

Furthermore it is not clear when you are trying these findChild/findChildren() calls. They will only work when the layout/sub-widgets you have added have actually been added onto their parent to find from. For example, if you have added them to form_layout but that has not yet been set as the layout on a widget you won't find them.

Hubris

@JonB Wow. You hit the nail on the head.

As it turned out I was using tabs in my GUI, where each tab was its own QWidget. The parent for the objects was that QWidget.

Many thanks. I have learned something new today. I still need to read up a bit more on Children and Parents to fully understand it.

Hubris

Here is the code snippet that best explains that:

        form_layout = QFormLayout()

        # Array of
        form_labels = [
            "Sales Order #",
            "Date Created",
            "Customer Name",
            "Contact #",
            "Email Address",
            "Template Name",
            "Size",
            "Material 1",
            "Material 2",
            "Additional",
            "Delivery",
            "Delivery Location",
            "Ship by Date",
            "Completed",
            "Shipped",
        ]

        item_names = [
            "so",
            "dc",
            "cn",
            "co",
            "ea",
            "tn",
            "sz",
            "m1",
            "m2",
            "ad",
            "de",
            "dl",
            "sd",
            "cp",
            "sh",
        ]

        # ---------- Create new tab ----------#
        tabs = QTabWidget()
        new_tab = QWidget()

        for itemnum in range(0, len(item_names) - 2):
            label = QLabel(form_labels[itemnum])
            qledit = QLineEdit()
            if item_names[itemnum] == "so" or item_names[itemnum] == "dc":
                qledit.setEnabled(False)
            label.setText(form_labels[itemnum])
            label.setObjectName(f"{item_names[itemnum]}_l")
            qledit.setObjectName(f"{item_names[itemnum]}_qle")
            # self.qledit.setParent(TabWidget)
            qledit.setFixedWidth(500)
            form_layout.addRow(label, qledit)

        cl_btn = QPushButton("Clear Fields")
        ce_btn = QPushButton("Create Entry")
        em_l = QLabel()

        btn_box = QHBoxLayout()
        btn_box.addWidget(em_l)
        btn_box.addWidget(ce_btn)
        btn_box.addWidget(cl_btn)

        form_layout.addRow(btn_box)
        new_tab.setLayout(form_layout)

        qle_by_name = new_tab.findChild(QLineEdit, "dc_qle")
        qle_by_name.setText("Test")

JonB

@Hubris said in PyQt 5: findChild returns None:

    new_tab.setLayout(form_layout)

    qle_by_name = new_tab.findChild(QLineEdit, "dc_qle")

Yes, this makes sense for finding the child widgets! :)