Refreshing Python AddIn combobox => TypeError: unbound method refresh() must be called with <class> instance as first argument (got nothing instead)?

Question

I have created a minimalist test Python AddIn using ArcGIS 10.4.1 for Desktop, with a toolbar and two comboboxes on it.

The code (TestAddIn_addin.py and config.xml) appears at the bottom of this question and when installed its interface is as below:

Clicking the down arrowhead next to:

• ComboBox1 offers two choices: item1 and item2
• ComboBox1 offers two choices: item3 and item4

This is a simplification of what I am wanting to do in practice, but in the test when I choose either item1 or item2 from ComboBox1 I want its onSelChange event to change the choices offered by ComboBoxClass2 to be A, B and C. The piece of code to do just that in the ComboBoxClass1 class is:

def onSelChange(self, selection):
    ComboBoxClass2.items = ["A","B","C"]
    ComboBoxClass2.refresh()

The above would seem to match the answer to Refreshing Combobox of Python Add-in of ArcPy?

However, when I test this Python AddIn from ArcMap with the Python window open I get an error:

Traceback (most recent call last): File "C:\Users\PolyGeo\AppData\Local\ESRI\Desktop10.4\AssemblyCache{EE92FFB8-6EED-46E8-83CC-6E8DD8A293BA}\TestAddIn_addin.py", line 14, in onSelChange ComboBoxClass2.refresh() TypeError: unbound method refresh() must be called with ComboBoxClass2 instance as first argument (got nothing instead)

It seems to want me to pass a value into ComboBoxClass2.refresh() and I have experimented with various stabs in the dark like ComboBoxClass2.refresh(self), ComboBoxClass2.refresh(ComboBoxClass2) and ComboBoxClass2.refresh(TestAddIn_addin.combobox1) but these only succeed in similar errors from the same line of code.

Does anyone know what value ComboBoxClass2.refresh() might be expecting as a parameter here?

ComboBoxClass2.refresh(self) moves the error to:

Traceback (most recent call last): File "C:\Users\PolyGeo\AppData\Local\ESRI\Desktop10.4\AssemblyCache{EE92FFB8-6EED-46E8-83CC-6E8DD8A293BA}\TestAddIn_addin.py", line 14, in onSelChange ComboBoxClass2.refresh(self) TypeError: unbound method refresh() must be called with ComboBoxClass2 instance as first argument (got ComboBoxClass1 instance instead)

C:\Temp\TestAddIn\Install\TestAddIn_addin.py:

import arcpy
import pythonaddins

class ComboBoxClass1(object):
    """Implementation for TestAddIn_addin.combobox1 (ComboBox)"""
    def __init__(self):
        self.items = ["item1", "item2"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'
    def onSelChange(self, selection):
        ComboBoxClass2.items = ["A","B","C"]
        ComboBoxClass2.refresh()
    def onEditChange(self, text):
        pass
    def onFocus(self, focused):
        pass
    def onEnter(self):
        pass
    def refresh(self):
        pass

class ComboBoxClass2(object):
    """Implementation for TestAddIn_addin.combobox2 (ComboBox)"""
    def __init__(self):
        self.items = ["item3", "item4"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'
    def onSelChange(self, selection):
        pass
    def onEditChange(self, text):
        pass
    def onFocus(self, focused):
        pass
    def onEnter(self):
        pass
    def refresh(self):
        pass

C:\Temp\TestAddIn\config.xml:

<ESRI.Configuration xmlns="http://schemas.esri.com/Desktop/AddIns" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Name>Python Addin</Name><AddInID>{ee92ffb8-6eed-46e8-83cc-6e8dd8a293ba}</AddInID><Description>New Addin</Description><Version>0.1</Version><Image /><Author>Untitled</Author><Company>Untitled</Company><Date>09/02/2016</Date><Targets><Target name="Desktop" version="10.1" /></Targets><AddIn language="PYTHON" library="TestAddIn_addin.py" namespace="TestAddIn_addin"><ArcMap>
    <Commands>
        <ComboBox caption="ComboBox1" category="Python Addin" class="ComboBoxClass1" id="TestAddIn_addin.combobox1" itemSizeString="WWWWWW" message="" rows="4" sizeString="WWWWWW" tip=""><Help heading="" /></ComboBox>
        <ComboBox caption="ComboBox2" category="Python Addin" class="ComboBoxClass2" id="TestAddIn_addin.combobox2" itemSizeString="WWWWWW" message="" rows="4" sizeString="WWWWWW" tip=""><Help heading="" /></ComboBox>
        </Commands>
    <Extensions>
        </Extensions>
    <Toolbars>
        <Toolbar caption="Toolbar" category="Python Addin" id="TestAddIn_addin.toolbar" showInitially="true"><Items><ComboBox refID="TestAddIn_addin.combobox1" /><ComboBox refID="TestAddIn_addin.combobox2" /></Items></Toolbar>
        </Toolbars>
    <Menus>
        </Menus>
    </ArcMap></AddIn></ESRI.Configuration>

Answer 1

To get this to work I appended two lines of code to the __init__ function of class ComboBoxClass2:

class ComboBoxClass2(object):
    """Implementation for TestAddIn_addin.combobox2 (ComboBox)"""
    def __init__(self):
        self.items = ["item3", "item4"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'

so that it became:

class ComboBoxClass2(object):
    """Implementation for TestAddIn_addin.combobox2 (ComboBox)"""
    def __init__(self):
        self.items = ["item3", "item4"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'
        global secondComboBox
        secondComboBox = self

and then in my ComboBoxClass1 class I changed:

def onSelChange(self, selection):
    ComboBoxClass2.items = ["A","B","C"]
    ComboBoxClass2.refresh("TestAddIn_addin.combobox2")

to:

def onSelChange(self, selection):
    secondComboBox.items = ["A","B","C"]
    secondComboBox.refresh()

In other words, and with the help of @NathanW, I made the instance of class ComboBox2 that I was using into a global called secondComboBox and then refreshed that within class ComboBox1.