calculateacres.py

import arcpy

# Get the feature name to work with
in_featureclass = arcpy.GetParameterAsText(0)

# Set local variables
field_Name = "ACRES"
field_Type = "DOUBLE"
field_Precision = 10 # total number of digits stored
field_Scale = 8 # number of decimal places


fieldList = arcpy.ListFields(in_featureclass, field_Name)
fieldCount = len(fieldList)
if fieldCount == 1:
    # field already exists
    arcpy.CalculateField_management(in_featureclass, field_Name, '!SHAPE.area@ACRES!', "PYTHON_9.3")
else:
    #field doesn't exist
    arcpy.AddField_management(in_featureclass, field_Name, field_Type, field_Precision, field_Scale)
    arcpy.CalculateField_management(in_featureclass, field_Name, '!SHAPE.area@ACRES!', "PYTHON_9.3")