How to work with layers in Combobox?

Question

I'm using the plugin builder to develop a plugin QGIS. I want the fields of the selected layer in combobox_1 to appear in combobox_5and fields of the selected layer in comboBox_2 to appear in comboBox_3 and comboBox_4.

I am new to python and PyQGIS. I do not have the methods defined in initGUI. How can I achieve this?

def initGui(self):
    """Create the menu entries and toolbar icons inside the QGIS GUI."""

    icon_path = ':/plugins/TheWorkflow/icon.png'
    self.add_action(
        icon_path,
        text=self.tr(u'The Workflow'),
        callback=self.run,
        parent=self.iface.mainWindow())

def unload(self):
    """Removes the plugin menu item and icon from QGIS GUI."""
    for action in self.actions:
        self.iface.removePluginVectorMenu(
            self.tr(u'&The Workflow'),
            action)
        self.iface.removeToolBarIcon(action)
    # remove the toolbar
    del self.toolbar

def run(self):
    """Run method that performs all the real work"""
    # show the dialog
    self.dlg.show()
    # Run the dialog event loop
    result = self.dlg.exec_()
    # See if OK was pressed
    if result:
        # Do something useful here - delete the line containing pass and
        # substitute with your code.
        pass

Answer 1

Just a small note that we should use QGIS builtin combobox:

• To choose a layer: QgsMapLayerComboBox https://qgis.org/api/classQgsMapLayerComboBox.html
• To choose a field in a layer: QgsFieldComboBox https://qgis.org/api/classQgsFieldComboBox.html

These QGIS objects manage to fill the combobox automatically with layers or to display fields according to a specific layer. And they provide a nice UI with some small icons:

Answer 2

You can try replacing your def run(self): function in your script for the one below which should do something you want:

def run(self):
    """Run method that performs all the real work"""
    # show the dialog
    self.dlg.show()

    # Get all loaded layers in the interface
    layers = self.iface.legendInterface().layers()
    # Create an empty list which we can populate
    layer_list = []
    # For every item (which we call "layer") in all loaded layers
    for layer in layers:
        # Add it to the list
        layer_list.append(layer.name())
    # Clear comboBox (useful so we don't create duplicate items in list)
    self.dlg.comboBox.clear()
    # Add all items in list to comboBox
    self.dlg.comboBox.addItems(layer_list)
    # Clear comboBox_2
    self.dlg.comboBox_2.clear()
    # Add all items in list to comboBox_2
    self.dlg.comboBox_2.addItems(layer_list)

    def layer_field():
        # Identify selected layer by its index
        selectedLayerIndex = self.dlg.comboBox.currentIndex()
        selectedLayer = layers[selectedLayerIndex]
        # Identify fields of the selected layer
        fields = selectedLayer.pendingFields()
        # Get field names of the fields
        fieldnames = [field.name() for field in fields]
        # Clear comboBox_5
        self.dlg.comboBox_5.clear()
        # Add field names to comboBox_5
        self.dlg.comboBox_5.addItems(fieldnames)

    def table_field():
        # Same comments as above
        selectedLayerIndex = self.dlg.comboBox_2.currentIndex()
        selectedLayer = layers[selectedLayerIndex]
        fields = selectedLayer.pendingFields()
        fieldnames = [field.name() for field in fields]
        self.dlg.comboBox_3.clear()            
        self.dlg.comboBox_3.addItems(fieldnames)
        self.dlg.comboBox_4.clear()
        self.dlg.comboBox_4.addItems(fieldnames)

    # When changing layer in comboBox, run the function "layer_field()"
    # to refresh the field names in the associated comboBoxes
    self.dlg.comboBox.currentIndexChanged.connect(layer_field)
    self.dlg.comboBox_2.currentIndexChanged.connect(table_field)

Answer 3

according to @Joseph's answer, if you want to populate your combobox with fields of vector layers, since raster layers does not have any fields of attribute you may get the error: AttributeError: 'QgsRasterLayer' object has no attribute 'fields'.

so for avoiding that you must equal your Layers with Only vector layers, like this:

layers = [layer for layer in iface.legendInterface().layers() if layer.type() == QgsMapLayer.VectorLayer]

instead of:

layers = iface.legendInterface().layers() 

good luck...