Tool validation - setting error message

Question

I am writing a tool within Python Toolbox that has two parameters:

param0 = arcpy.Parameter(
          name="a",
          displayName="A",
          datatype="GPFeatureLayer",
          parameterType="Required",
          direction="Input")

param1 = arcpy.Parameter(
          name="b",
          displayName="B",
          datatype="GPFeatureLayer",
          parameterType="Required",
          direction="Input")

params = [param0, param1]

I can´t figure out which if statement I should use in def updateMessages(self, parameters):. Here is a step-by-step what´s going wrong when I run the tool:

1. I choose a layer from the TOC as param0. My validation code is
   if not parameters[0].hasBeenValidated: ... check e.g. if param0 has some attribute field ... ... it turns out that such field doesn´t exist, so... parameters[0].setErrorMessage("No field!")
   The red error icon appears next to its label with the message "No field!" - everything is working as expected.
2. But then when I choose the second layer from the TOC as param1 (it has no validation code) the param0´s red icon disappears like there is no error with the param0! I am sure that the problem is with if not parameters[0].hasBeenValidated: as I don´t quite understand how it works. I tested several code variants with .hasBeenValidated, .altered etc. but with no success.

To be short:

1. I would need the red error icon to be displayed as long as there is an error with the param0 no matter what I do with other parameters and
2. (if possible - not mentioned above) ensure that param0´s validation performs only when the user changes the value of this parameter.

Could you give me a hint how to accomplish this?

Answer 1

If you use altered method of the Parameter object, your simple validation routine should work fine, e.g.,

def updateMessages(self):
    """Modify the messages created by internal validation for each tool
    parameter.  This method is called after internal validation."""
    if  parameters[0].altered:
    #check e.g. if param0 has some attribute field ...
    #... it turns out that such field doesn´t exist, so...
        if not "Sought_Field" in [f.name for f in arcpy.ListFields(parameters[0].value)]:
            parameters[0].setErrorMessage("No such field!")