Disabling automatic selection in QgsMapLayerComboBox

Question

I build a plugin. It has a lot of comboboxes (QgsMapLayerComboBox). They are all empty when no project is loaded. When I open a project in QGIS, a layer is automatically shown in all comboboxes. All is the same layer.

I don't want a layer is selected. How can I avoid that?

A sample form and script for QGIS Python Editor:

from qgis.PyQt.QtCore import * 
from qgis.PyQt.QtGui import * 
from qgis.PyQt.QtWidgets import * 
  
class Window(QMainWindow):
  
    def __init__(self):
        super(Window, self).__init__()
        self.setGeometry(250, 250, 300, 100)
        self.setWindowTitle("Test")
        
        cb1 = QgsMapLayerComboBox(self)
        cb2 = QgsMapLayerComboBox(self)

        layout = QVBoxLayout()
        layout.addWidget(cb1)
        layout.addWidget(cb2)
        
        contents = QWidget()
        contents.setLayout(layout)
        self.setCentralWidget(contents)

window = Window()
window.setWindowFlags(window.windowFlags() | Qt.WindowStaysOnTopHint)  
window.show()

Answer 1

Use this for all QgsMapLayerComboBox.

maplayercombobox.setAllowEmptyLayer(True)
maplayercombobox.setCurrentIndex(0)

If you set an index after using setAllowEmptyLayer(True), since combobox has a selected item (empty item), any layer is not automatically shown in the comboboxes when you load a project.

---

It can be cumbersome to set those for each combobox. Instead, find all QgsMapLayerComboBox of the widget and use the commands above inside a for loop.

for cb in self.findChildren(QgsMapLayerComboBox):
    cb.setAllowEmptyLayer(True)
    cb.setCurrentIndex(0)