arcgis python parameter validation

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."""
    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):
    """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:
        try:
            desc = arcpy.Describe(self.params[0].value)
            shapeType = desc.shapeType
            sr = desc.spatialReference
            gcsOrMercator = False if not sr.GCSName and sr.PCSName.upper().find("MERCATOR") == -1 else True

            if shapeType.upper() == "POINT":
                self.params[1].filter.list = ["POINT_X_Y_Z_M"]
            elif shapeType.upper() == "MULTIPOINT":
                self.params[1].filter.list = ["CENTROID", "PART_COUNT", "EXTENT"]
            elif shapeType.upper() == "POLYLINE":
                if desc.hasZ:
                    lineList = ["LENGTH", "LENGTH_GEODESIC", "LENGTH_3D", "LINE_START_MID_END", "CENTROID", "CENTROID_INSIDE", "PART_COUNT", "POINT_COUNT", "LINE_BEARING", "EXTENT"]
                else:
                    lineList = ["LENGTH", "LENGTH_GEODESIC", "LINE_START_MID_END", "CENTROID", "CENTROID_INSIDE", "PART_COUNT", "POINT_COUNT", "LINE_BEARING", "EXTENT"]
                self.params[1].filter.list = lineList
                if gcsOrMercator:
                    lineList.remove("LENGTH")
                    if desc.hasZ:
                        lineList.remove("LENGTH_3D")
                    self.params[1].filter.list = lineList
            elif shapeType.upper() == "POLYGON":
                polyList = ["AREA", "AREA_GEODESIC", "PERIMETER_LENGTH", "PERIMETER_LENGTH_GEODESIC", "CENTROID", "CENTROID_INSIDE", "LINE_START_MID_END", "PART_COUNT", "POINT_COUNT", "EXTENT"]
                self.params[1].filter.list = polyList
                if gcsOrMercator:
                    polyList.remove("AREA")
                    polyList.remove("PERIMETER_LENGTH")
                    self.params[1].filter.list = polyList

            if self.params[1].value:
                self.params[5].parameterDependencies = [0]
                self.params[5].schema.clone = True
                newFields = []
                propDict = {"POINT_X_Y_Z_M":            ["POINT_X",
                                                         "POINT_Y",
                                                         "POINT_Z",
                                                         "POINT_M"],
                            "PART_COUNT":               ["PART_COUNT"],
                            "CENTROID":                 ["CENTROID_X",
                                                         "CENTROID_Y",
                                                         "CENTROID_Z",
                                                         "CENTROID_M"],
                            "EXTENT":                   ["EXT_MIN_X",
                                                         "EXT_MIN_Y",
                                                         "EXT_MAX_X",
                                                         "EXT_MAX_Y"],
                            "POINT_COUNT":              ["PNT_COUNT"],
                            "LINE_START_MID_END":       ["START_X",
                                                         "START_Y",
                                                         "START_Z",
                                                         "START_M",
                                                         "MID_X",
                                                         "MID_Y",
                                                         "MID_Z",
                                                         "MID_M",
                                                         "END_X",
                                                         "END_Y",
                                                         "END_Z",
                                                         "END_M"],
                            "LINE_BEARING":             ["BEARING"],
                            "CENTROID_INSIDE":          ["INSIDE_X",
                                                         "INSIDE_Y",
                                                         "INSIDE_Z",
                                                         "INSIDE_M"],
                            "LENGTH":                   ["LENGTH"],
                            "PERIMETER_LENGTH":         ["PERIMETER"],
                            "AREA":                     ["POLY_AREA"],
                            "LENGTH_GEODESIC":          ["LENGTH_GEO"],
                            "AREA_GEODESIC":            ["AREA_GEO"],
                            "LENGTH_3D":                ["LENGTH_3D"],
                            "PERIMETER_LENGTH_GEODESIC":["PERIM_GEO"],
                            }
                currentFields = [x.name for x in desc.fields]
                for prop in str(self.params[1].value).split(";"):
                    try:
                        for field in propDict[prop]:
                            if not field in currentFields:
                                newField = arcpy.Field()
                                newField.type, newField.name = "DOUBLE", field
                                newFields.append(newField)
                    except:
                        pass
                self.params[5].schema.additionalFields = newFields
        except:
            pass
    return


  def updateMessages(self):
    """Modify the messages created by internal validation for each tool
    parameter.  This method is called after internal validation."""
    return