How to use the List field function for a toolbox script?

Question

I am trying to write a simple script for a toolbox, similar to the Addfield_datamanagement toolbox. The script adds a new field to a table of a feature class or a shapefile with attribute tables. So far so good.

# Import system modules
import arcpy
#Workspace
arcpy.env.workspace="C:\\Users\\xxx.gdb"
# Set feature class
inputFC = arcpy.GetParameterAsText(0)
# Get List of Fieldnames
inputString =arcpy.GetParameterAsText(1)
inputString =arcpy.ValidateFieldName(inputString,arcpy.env.workspace)
fieldList =inputString.split(";")
fieldType = arcpy.GetParameterAsText(2)
for fieldName in fieldList:
          arcpy.AddField_management(inputFC , fieldName, fieldType)
arcpy.AddMessage("Field generated:" + fieldName)
arcpy.AddMessage ("Script executed")

So this seems to work, but now I want to check first if the fieldname already exists before it´s created and print the existing name! I thought about the list fields together with the built-in len() function:

if len(arcpy.ListFields(inputFC,?)==1: #not sure if inputFC is right?
             arcpy.AddMessage("Field already exists"+fieldName)

Not sure about the syntax of the ListFields command and how to integrate it in the code above! So, any help is welcome and appreciated!

Answer 1

ListFields returns a list of field objects, and each of these objects has attributes associated with it.

So the structure looks like this:

field_list = [field object1: field.name, field.type, etc...;
              field object2: field.name, field.type, etc...;
             ]

You'll probably want to access the .name attribute of each field object to determine if it matches any name in the variable fieldList.

First, generate a list of field names:

existingFields = [] # empty list
for field in arcpy.ListFields(inputFC): # iterate over fields
    existingFields.append(field.name)   # add the attribute name to list for each field

Next, compare the list of existing fields with the list of fields the user wants to add:

duplicateFields = set(existingFields) & set(fieldList)

if len(duplicateFields) == 0:
    carry on with adding fields
else:
    arcpy.AddMessage('Field(s) already exist: ' + duplicateFields)