I would like to expand the combobox field in a form layout, but I just can’t seem to find the right combination of spells to achieve this. Specific documentation on practical layout management seems a bit thin on the ground – I don’t get a lot of mileage out of the doc.qt.io site.

This image shows what I am trying to achieve:

The label and combo box are contained in a QFormLayout, which in turn is contained within a QVBoxLayout.

What actually happens is that the label/combo occupies the left hand side, with the combo NOT extended to the size of the QVBoxLayout width.

Here is my code:

import sys
from PyQt6.QtWidgets import  QApplication, QComboBox, QDialog, QWidget, QVBoxLayout,  QFormLayout, QTableView,  QDialogButtonBox, QHeaderView, QLabel, QSizePolicy
from PyQt6.QtCore import Qt

class Dialog(QDialog):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setObjectName("Dialog")
        self.resize(500, 300)
        
        self.verticalLayoutWidget = QWidget(self)
        self.verticalLayoutWidget.resize(495, 295)
        self.verticalLayout = QVBoxLayout(self.verticalLayoutWidget)
        self.verticalLayout.setContentsMargins(5, 5, 5, 5)
        
        self.tableView = QTableView(parent=self.verticalLayoutWidget)
        self.tableView.resizeColumnsToContents()
        self.tableView.horizontalHeader().setSectionResizeMode(QHeaderView.ResizeMode(1))
        self.tableView.SelectionMode(1)  
        self.verticalLayout.addWidget(self.tableView)

        self.myCombo = QComboBox()
        self.items = ['N/A', 'Amphibian','Bird','Fish','Invertebrate','Mammal', 'Reptile']
        self.myCombo.addItems(self.items)
        #self.myCombo.currentTextChanged.connect(self.model.customSlot)
        self.myCombo.setSizePolicy(QSizePolicy.horizontalStretch())

        self.formLayout = QFormLayout()
        self.formLayout.setObjectName("formLayout")
        self.formLayout.setContentsMargins(5, 5, 5, 5)
        self.label = QLabel('Category', parent=self.verticalLayoutWidget)
        self.label.setObjectName("categoriesLabel")
        self.formLayout.setWidget(0, QFormLayout.ItemRole.LabelRole, self.label)
        self.formLayout.setWidget(0, QFormLayout.ItemRole.FieldRole, self.myCombo)
        self.formLayout.setFormAlignment(Qt.AlignmentFlag.AlignLeft)  # the documentation specifies Qt.AlignLeft
        self.formLayout.setFieldGrowthPolicy(QFormLayout.FieldGrowthPolicy.ExpandingFieldsGrow)

        self.verticalLayout.addLayout(self.formLayout)
        #self.myCombo.setModelColumn(1)
                
        self.buttonBox = QDialogButtonBox(parent=self.verticalLayoutWidget)
        self.buttonBox.setStandardButtons(QDialogButtonBox.StandardButton.Cancel|QDialogButtonBox.StandardButton.Ok)
        self.buttonBox.setObjectName("buttonBox")
        self.verticalLayout.addWidget(self.buttonBox)
        self.buttonBox.rejected.connect(self.close)
        #self.buttonBox.accepted.connect(self.model.showModel)
        

if __name__ == '__main__':        
    app = QApplication(sys.argv)
    ui = Dialog()
    ui.show()
    app.exec()

I was hoping that the following lines I used might help :

self.myCombo.setSizePolicy(QSizePolicy.horizontalStretch())
self.formLayout.setFieldGrowthPolicy(QFormLayout.FieldGrowthPolicy.ExpandingFieldsGrow)

Does anyone have any ideas on how I might achieve this

I have tried dozens of combinations of setSizePolicy on the combo box and on the form layout but invariably I generate errors