How can I generalize "field name" variable?

Question

Here is my code:

import arcpy
import math
arcpy.env.workspace = "C:\RG\Kings County\Test.gdb"
fcs = arcpy.ListFeatureClasses("NVF_*")
try:
 for fc in fcs:
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 if row.Field_ID == 1:
 rows.deleteRow(row)
 del rows, row
# This code adds up several fields like Ratio_NDVI, Ratio_Acres etc.
 arcpy.AddField_management (fc, "Ratio_NDVI", "float")
 arcpy.AddField_management (fc, "Ratio_Area", "float")
 arcpy.AddField_management (fc, "Final", "integer")-
 # This code calculates area in square meter using existing datum coordinates of the vector file
 arcpy.AddField_management(fc, "NewArea", "DOUBLE")
 geometryField = arcpy.Describe(fc).shapeFieldName
 cursor = arcpy.UpdateCursor(fc)
 for row in cursor:
 AreaValue = row.getValue(geometryField).area
 row.setValue("NewArea", AreaValue)
 cursor.updateRow(row)
 del row, cursor
 # This code converts square meter into acres
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 row.NewArea = (row.NewArea*0.00024711)
 rows.updateRow(row)
 del rows, row
 # This code calculates ratio of NDVI of child_fields to the average NDVI of parent field
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 if row.NDVI090606 == 0: 
 row.Ratio_NDVI = (row.M090606/row.M090606) 
 else:
 row.Ratio_NDVI = (row.M090606/row.NDVI090606) 
 rows.updateRow(row)
 del rows, row
 # This code calculates the ratio of acres of child fields to their corresponding parent field
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 row.Ratio_Area = (row.NewArea/row.Acres)
 rows.updateRow(row)
 del rows, row
 print ("Completed calculating ratio of acres")
 # This code finds that field that has either Ratio_NDVI higher than 1.5 to its parent #field or less than 0.5 to its parent field and Ratio_Acres higher than 0.05. Then #comments this field as "Change"
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 if (row.Ratio_Area > 0.05 and row.Ratio_NDVI > 1.5):
 row.Final = 1
 rows.updateRow(row)
 if (row.Ratio_Area > 0.05 and row.Ratio_NDVI < 0.5):
 row.Final = 1
 rows.updateRow(row)
 del rows, row
except Exception as e:
 print e

In this code, how can I generalize the field name variable (row.NDVI090906). In above, NDVI090906 is a field name, where this field name follows the pattern of the name of the file. For example, if the file name is 'NVF_090906' the corresponding field name is NDVI090906. How can I make this field name as a variable so that when it loops through, it automatically performs computation of field names of other feature classes.

• Get Spatial: Is the end of the file name always 6 numbers? Can it include numbers? Give us some details about the file name. It may be a simple matter of parsing it apart into separate variables, but we need to know what may or may not change about the name first.

Answer 1

Use row.getValue('FieldName') instead of row.FieldName. Using getValue lets you pass in a variable name for a field name instead of hard-coding it.

 nvdi_field = 'NDVI090606'
 m9_field = 'M090606'
 ratio_field = 'Ratio_NDVI'
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 if row.getValue(nvdi_field) == 0: 
 row.setValue(ratio_field, (row.getValue(m9_field)/row.getValue(m9_field)))
 else:
 row.setValue(ratio_field, (row.getValue(m9_field)/row.getValue(nvdi_field)))
 rows.updateRow(row)

You can also use getattr(object, attribute) and setattr(object, attribute, value) on any object in Python to get a similar behavior, allowing you to program in the field name to get/set.

• Kuma: But 'NDVI090606' is the field name associated to feature class 'NVF_090606'. When the 'for' loop loops for another feature class, e.g., NVF_052013, it flags error to ndvi_field. It is because, the field name for that feature class is NDVI052013. How to handle this?
• Jason Scheirer: Something like nvdi_field = "NVDI" + fcnamelast

Answer 2

Give this code a shot. @Jason had the right idea with using the Get and Set values.

I took your code and did three things.

1. I pulled the feature class name, split off the prefix, and used the second part as part of the NDVI and M variables.
2. Replaced instances of hard-coding field names with variable references. This makes it easier if you need to change a field name, and reduces repetition.
3. Combined loops that were performing separate functions. Reduces complexity of code, and multiple cycling through of rows.

I added comments to the code as necessary. I parsed the feature class name based on your filter for creating the list. You use an underscore (_) to separate the prefix of the name from the portion you want to use as part of the field names. I use that as the object to split the string. The 2nd half of the split is concatenated with a new prefix to form the two new field name variables that are unique to each featureclass.

import arcpy
arcpy.env.workspace = "C:\RG\Kings County\Test.gdb"
fcs = arcpy.ListFeatureClasses("NVF_*")
try:
 for fc in fcs:
 fcnamelast = fc.split('_',1)[1]
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 if row.Field_ID == 1:
 rows.deleteRow(row)
 del rows, row
 # This code adds up several fields like Ratio_NDVI, Ratio_Acres etc.
 # Create variables containing field names first, allowing ease of reuse
 fieldndvi = "Ratio_NDVI"
 fieldndvichild = "NDVI" + fcnamelast
 fieldm9 = "M" + fcnamelast
 fieldratarea = "Ratio_Area"
 fieldfinal = "Final"
 fieldnewarea = "NewArea"
 arcpy.AddField_management (fc, fieldndvi, "float")
 arcpy.AddField_management (fc, fieldratarea, "float")
 arcpy.AddField_management (fc, fieldfinal, "integer")
 arcpy.AddField_management(fc, fieldnewarea, "DOUBLE")
 # This code calculates area in square meter using existing datum coordinates of the vector file
 #Convert square meter to acres
 geometryField = arcpy.Describe(fc).shapeFieldName
 cursor = arcpy.UpdateCursor(fc)
 for row in cursor:
 AreaSqMet = row.getValue(geometryField).area
 AreaAcres = AreaSqMet * 0.00024711
 row.SetValue(fieldnewarea, AreaAcres)
 cursor.updateRow(row)
 del row, cursor
 # This code calculates ratio of NDVI of child_fields to the average NDVI of parent field
 # This code calculates the ratio of acres of child fields to their corresponding parent field
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 ndvichildval = row.getValue(fieldndvichild)
 m9childval = row.getValue(fieldm9)
 if ndvichildval == 0: 
 row.SetValue(fieldndvi, (m9childval/m9childval)) 
 else:
 row.SetValue(fieldndvi, (m9childval/ndvichildval))
 #Area ratio calculation
 areaacresval = row.getValue("Acres")
 areanewval = row.getValue(fieldnewarea)
 row.SetValue(fieldratarea,areanewval/areaacresval)
 print ("Completed calculating ratio of acres") 
 rows.updateRow(row)
 del rows, row
 # This code finds that field that has either Ratio_NDVI higher than 1.5 to its parent #field or less than 0.5 to its parent field and Ratio_Acres higher than 0.05. Then #comments this field as "Change"
 rows = arcpy.UpdateCursor(fc)
 for row in rows:
 ratareaval = row.getValue(fieldratarea)
 ratndvival = row.getValue(fieldndvi)
 if (ratareaval > 0.05 and ratndvival > 1.5):
 row.SetValue(fieldfinal, 1)
 rows.updateRow(row)
 if (ratareaval > 0.05 and ratndvival < 0.5):
 row.SetValue(fieldfinal, 1)
 rows.updateRow(row)
 del rows, row
except Exception as e:
 print e