Prepopulation of "Field" parameter if the field exist using ArcPy?

Question

I have a python toolbox with a tool that takes two arguments. First parameter is GPFeatureLayer and the second is Field of Integer type. I would like to set the second parameter to a specific field if it exist within the layer but I am unable to figure out a way to do it correctly.

My code looks like this:

def getParameterInfo(self):
    param0 = arcpy.Parameter(
        displayName="Input lines",
        name="in_lines",
        datatype="GPFeatureLayer",
        parameterType="Required",
        direction="Input")
    param0.filter.list = ["Polyline"]

    param1 = arcpy.Parameter(
        displayName="ID source",
        name="in_id_source",
        datatype="Field",
        parameterType="Required",
        direction="Input")
    param1.filter.list = ["Integer"]
    param1.parameterDependencies = [param0.name]
params = [param0, param1]
return params

Later in the code I would like to set parameter 1 to field "OID_SOURCE" as a default value if it exists. However, the following code is not working:

def updateParameters(self, parameters):
    if parameters[0].value:
        fields = arcpy.Describe(parameters[0].valueAsText).fields
        index = None
        for i in range(0, len(fields)):
            if fields[i].name == "OID_SOURCE":
                index = i
        parameters[1].value = fields[index]
    return

This makes the parameter 1 always empty. I tried using this:

parameters[1].value = fields[index].name

But that yelds error "Invalid field type". I do understand why the version with "fields[index].name" is not working as I am trying to asssing String to a Field.

But I am wondering is there any way preset the the parameter with specific field if it exists?

Answer 1

There is a workaround that maybe you won't like it, but I will post it anyway:

Instead of using "Field" as the second parameter, I use "GPString"

def getParameterInfo(self):

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

    param1 = arcpy.Parameter(
    displayName="ID source",
    name="in_id_source",
    datatype="GPString",
    parameterType="Required",
    direction="Input")

    return [param0, param1]

def updateParameters(self, parameters):

    if parameters[0].altered:
        fields = arcpy.ListFields(parameters[0].value)
        fil = []
        for field in fields:
            if field.type == "Integer":
                fil.append(field.name)
                if field.name == "OID_SOURCE":
                     parameters[1].value = field.name

        parameters[1].filter.type = "ValueList"
        parameters[1].filter.list = fil 

    return

Then in the execute you can get the Field:

def execute(self, parameters, messages):

   inFC = parameters[0].value
   f = parameters[1].value

   fObj = None #The Field you want

   fields = arcpy.ListFields(inFC)
   for field in fields:
      if field.name == f:
         fObj = field   

   return

Answer 2

If you know the field name is "OID_SOURCE" and just want to check that it exists before setting it as parameters[1].value then check a list of the field names and if found set your parameter:

fields = {x.name for x in arcpy.Describe(parameters[0].valueAsText).fields}
if "OID_SOURCE" in fields:
    parameters[1].value = "OID_SOURCE"