I'm trying to build a toolbox, and I would like users to be able to select two possible values from two lists.
The first list is stored in a JSON, that I access using HTML.
The second list is also a JSON, but its HTML varies depending on the first one selected.
For such, I trying to code a Tool Validator.

Here's the code:

import arcpy
import requests

class ToolValidator(object):
    def __init__(self):
        self.params = arcpy.GetParameterInfo()   
    def initializeParameters(self):
    def updateParameters(self):
        data = requests.get(u'-first.html-').json()
        values = [d['facility_name'] for d in data if 'facility_name' in d]
        self.params[0].filter.list = values
    def updateMessages(self):
        if self.params[0].altered:
                facility = self.params[0].value
                data2 = requests.get(u'-second.html-{}'.format(facility)).json()
                values2 = [d['task_code'] for d in data2]
                self.params[1].filter.list = values2

It works fine for the most part, but for some list items, I get an error (Value Error: FilterObject: invalid list value) and do not get the second list.

At first, I thought it could be an encoding problem since the HTML can have values with accents. However, I tested several different values, but I get this error in some items that do not have special characters, and I don't have any error in other items that do have special characters.

params[0] and params[1] are texts.

I have tried a similar code directly on the python terminal and it works perfectly. I only get errors using the toolbox.

Any ideas on what am I doing wrong?

  • 1
    I assume without seeing the original html files that you refer to it's almost impossible to answer this question? But as a fudge, are these html files changing, if not could you hardwire the choices?
    – Hornbydd
    Sep 21, 2021 at 17:43
  • The first json is fixed, but the second one changes depending on the option selected. Sep 21, 2021 at 17:46
  • Are you actually using a Python Toolbox (*.pyt) or are you instead trying to apply tool validation to a Python Script tool in a standard toolbox (*.tbx)?
    – PolyGeo
    Oct 6, 2021 at 22:37

1 Answer 1


A colleague was able to figure out the issue: some options, for an unknown, had an empty value u'' in the values2 list. That was the problem, because the ArcGIS wouldnt recognize this empty as a valid item for the filter list, and returned an error.

We added a piece of code to remove these empty values that would appear in some options, and everything worked out.

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