ArcGIS Toolbox Python Script Validation: extracting field names from a multi-value list of feature classes

Question

I am writing a python script within an ArcGIS toolbox (not a Python Toolbox), that takes as input, a multivalue parameter of feature classes from which I would like to populate a second parameter with a drop down list of fields from the first feature class in the multivalue list. When using a single feature class as input, the field parameter can simply be set using the "Obtained from" functionality in the "Parameters" tab, but not so with a multivalue input. I am attempting to perform the operation using the validation classes instead. Below is my attempt using the "updateParameters" class. The validation doesn't fail, but doesn't produce a list of fields as expected. Any thoughts?

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[0].value:
      self.params[1].filter.list = arcpy.ListFields(self.params[0].valueAsText.split(";")[0])
  return

Edit - I’ve also tried:

[f.name for f in arcpy.ListFields(self.params[0].valueAsText.split(";")[0])]

With no success

Answer 1

This validation works:

  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[0].value:
      lyr=self.params[0].valueAsText.split(";")[0]
      aList=[f.name for f in arcpy.ListFields(lyr)]
      self.params[1].value=aList
    return

if you launch tool from current mxd. Parameter of interest:

However, using hidden parameter (2) that is taking 1st entry from multiple layers list:

  def initializeParameters(self):
    """Refine the properties of a tool's parameters.  This method is
    called when the tool is opened."""
    self.params[2].enabled=0
    self.params[2].category = "Information Only"
    return

  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[0].value:
      lyr=self.params[0].valueAsText.split(";")[0]
      self.params[2].value=lyr  
    return

Gives you more flexibility, because you'll be able to use filter on field types on parameter1 while obtaining fields from hidden.