I'm new to ToolValidator for a script tool and I'd like to make two checks which then adds an error if either rule is violated. I need to make sure parameter 2 and 3 (feature classes) are within parameter 1 (feature dataset). The tool will fail (roughly 3/4 of the way through) if the feature classes are not contained in the feature dataset.

Also, the tool needs to display an error is the feature class is empty.

I know Tool Validator is the way to go. I don't have much and am willing to read stuff. Also, this doesn't work, but I've managed to piece it together with some other posts.

import arcpy
class ToolValidator(object):
  """Class for validating a tool's parameter values and controlling
  the behavior of the tool's dialog."""

def __init__(self):
    """Setup arcpy and the list of tool parameters."""
    import arcpy    
    self.params = arcpy.GetParameterInfo()

def initializeParameters(self):
    """Refine the properties of a tool's parameters.  This method is
    called when the tool is opened."""

def updateParameters(self):
    """Modify the values and properties of parameters before internal
    validation is performed.  This method is called whenever a parameter
    has been changed."""
    if self.params[3].value==True:
        rows = arcpy.SearchCursor(fc)
    if rows.next():

def updateMessages(self):
"""Modify the messages created by internal validation for each tool
parameter.  This method is called after internal validation."""
    if self.params[3].hasBeenValidated:
        self.params[3].setErrorMessage('Your feature class is empty. 300 Tool will not fail.')

1 Answer 1


Tool validators are not easy to program, there's no debugging or text output so all you get is does/doesn't work. I notice that your if rows.next(): is on an incorrect indent level which could be why it isn't working. I also notice you're equating the self.params[3].value==True: where the value should be a path to a feature class.. not sure what's going on there.

As I see your problem you want to know if parameter 2 & 3 start with parameter 1 which should be string manipulation and to test if parameter 2 & 3 both contain some features. This should get you moving in the right direction..

# I have expanded the terms to make it easier to follow the logic
# make them all uppercase to make comparison easier
P1_val = self.params[1].value.upper()
P2_val = self.params[2].value.upper()
P3_val = self.params[3].value.upper()

P1_len = len(P1_val) # the length of the first parameter
# Python substrings are specified like this: source[start:end]
# so "abcd"[0:2] = "ab".. makes this next bit easier to read
if P2_val[0:P1_len] == P1_val and P3_val[0:P1_len] == P1_val:
    # both p2 and p3 contain p1
    # move forward to getting the counts
    cntObj = arcpy.GetCount_management(P2_val) # GetCount returns an object
    P2_cnt = int(cntObj.getOutput(0))          # get the count from the object

    cntObj = arcpy.GetCount_management(P3_val) # GetCount returns an object
    P3_cnt = int(cntObj.getOutput(0))          # get the count from the object

    if P3_cnt != 0 and P2_cnt !=0:
        # all good!

This will find if both parameter 2 (P2) and parameter 3 (P3) are inside parameter 1 (P1) and that both have feature counts not equal to 0 (can't be negative).

The next question is why does your script fall over about 3/4 the way through if they're not in the same feature dataset?

  • Topology comes in 3/4 of the way through. Which means another person would not know their inputs are incorrectly chosen until topology.
    – Jennifer
    May 20, 2015 at 14:06
  • As for your other concerns, ie. What's going on there?, I usually manage to piece together scripts reading a whole lot of help menu and stack exchange. Eventually, I find enough information and try enough things that I get it to work. But tool validator doesn't have many questions. I did find a post about empty feature classes and trying to use a search cursor, and another post used a method that did an update after the parameter was validated by tool validator. I knew what I had was wrong, but it was a start. But like you said, tool validator will not give errors if things do not work.
    – Jennifer
    May 20, 2015 at 14:10
  • Yes Jennifier, you wont find much on tool validators because they're such a pain to work with (as you've found out no doubt); it's much easier (to debug) to skip the validator and do the testing in the script. You're certainly learning a lot doing what you're doing so keep up the good work! That's how I learned python for ArcGis. One trick I learned is to do the validation first in the script then once that was working migrate it to a validator (change the object names of course). What python editor do you use to write your code? a better editor will show inconsistent indent and save you time. May 20, 2015 at 22:47

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