Filtering parameters in ArcPy Python script tool avoid repeating selecting same value in sequence parameters?

Question

I am trying to make a tool in which there are many sequence parameters (based on the same field of a feature class), and each of them is multi value to make a user choose from a check a list one or more value from values in this field.

If the same field's values I need to use in sequence parameters, but I want to avoid the duplication in selecting values in these sequence parameters, so how to adjust the validation code, so that if the user select definite value(s) in a parameter, that or these values can not be selected in the following parameter, unless that or these values are unselected in the previous parameters?

For example, if in a parameter the user select the value "Canada" based on a field for countries' names, I want to avoid the repeating of selecting the value "Canada" in a following parameter, that is also based on the same field for countries' names, unless the user return to the previous parameter and unselect or uncheck the value "Canada".

---

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

    def initializeParameters(self):

        """Refine the properties of a tool's parameters.  This method is
        called when the tool is opened."""
        return

    def updateParameters(self):
        if not params[0].value:
            """Modify the values and properties of parameters before internal
            validation is performed.  This method is called whenever a parameter
            has been changed."""
            global previousProjectPath3
            previousProjectPath3 = ""
            if self.params[1].value and self.params[3].value:
                self.params[4].filter.list = sorted({row[0] for row in arcpy.da.SearchCursor(self.params[1].value, self.params[3].value.value) if row[0]})
            if self.params[1].value and self.params[3].value:
                self.params[5].filter.list = sorted({row[0] for row in arcpy.da.SearchCursor(self.params[1].value, self.params[3].value.value) if row[0]})        

    def updateMessages(self):
        """Modify the messages created by internal validation for each tool
        parameter.  This method is called after internal validation."""
        if params[0].altered:
            path = params[0].valueAsText
            global previousProjectPath3
            if previousProjectPath3 != path:
                previousProjectPath3 = path

Answer 1

I have yet to find a good solution to this problem. Normally you could store the value at the class level, but I've found ArcGIS creates multiple instances of the same python class which complicates where the value actually got stored.

The solution that I've used is to create a global variable that retains the value of the previous selection.

I found setting the global variable in the updateParameters function works well. Then performing the checks for previous selected values in the updateMessages. To create a global variable you need to call it as such everytime you intend to use it (e.g. global previousProjectPath3). Make sure to clear it in your def execute code because the value will be stored and used by any other tool in your toolbox that uses it.

def updateParameters(self, parameters):

    """Modify the values and properties of parameters before internal
    validation is performed.  This method is called whenever a parameter
    has been changed."""
    if not parameters[0].value:
        global previousProjectPath3
        previousProjectPath3 = ""

def updateMessages(self, parameters):
        """Modify the messages created by internal validation for each tool
        parameter.  This method is called after internal validation."""
        if parameters[0].altered:
            path = parameters[0].valueAsText
            global previousProjectPath3
            if previousProjectPath3 != path:
                previousProjectPath3 = path

EDIT based on code provided.

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

    def initializeParameters(self):

        """Refine the properties of a tool's parameters.  This method is
        called when the tool is opened."""
        return

    def updateParameters(self):
        if not params[0].value:
            """Modify the values and properties of parameters before internal
            validation is performed.  This method is called whenever a parameter
            has been changed."""
            global previousValues
            previousValues = []
            if self.params[1].value and self.params[3].value:
                self.params[4].filter.list = sorted({row[0] for row in arcpy.da.SearchCursor(self.params[1].value, self.params[3].value.value) if row[0]})
            if self.params[1].value and self.params[3].value:
                self.params[5].filter.list = sorted({row[0] for row in arcpy.da.SearchCursor(self.params[1].value, self.params[3].value.value) if row[0]})        

    def updateMessages(self):
        """Modify the messages created by internal validation for each tool
        parameter.  This method is called after internal validation."""
        if self.params[4].altered:
            selectedValue = self.params[4].value
            global previousValues
            if selectedValue in previousValues:
                self.params[4].setWarningMessage("Already selected")
            else:
                previousValues.append(selectedValue)