I am attempting to populate fields on two different feature classes with different strings that I have stored in a list. I would like to just put the strings and feature classes into a for loop, but it has not been working. Here is my current script that makes the most sense to me:

def protectedlandslayer(PAD, NCED, counties, protectedFC):

    # set local variables in lists to prepare for looping
    ProtectedLands = [PAD, NCED]
    layers = ["pad", "nced"]
    county_lyr = arcpy.MakeFeatureLayer_management(counties, "counties_lyr")

    # select protected lands that intersect PA county boundaries and copy - populate
    # list with output feature classes to use in add field loop
    temp_featureclasses = []
    for FC, layer in zip(ProtectedLands, layers):
        outFC = "in_memory" + "\\" + layer
        layer1 = arcpy.MakeFeatureLayer_management(FC, layer)
        selection = arcpy.SelectLayerByLocation_management(layer1, "INTERSECT", county_lyr)
        protected_fc = arcpy.CopyFeatures_management(selection, outFC)

    # add field to each dataset and populate with data source
    for name, f in zip(layers, temp_featureclasses):
    arcpy.AddField_management(f, "data_src", "TEXT", "", "",
    4, "Data Source", "", "", "")
        expression = str(name)
        arcpy.CalculateField_management(f, "data_src",
    expression, "PYTHON_9.3")

With this script, I am getting the following error:

ExecuteError: ERROR 000539: Error running expression: n  Traceback (most recent call last):   File "<expression>", line 1, in <module> NameError: name 'n' is not defined  Failed to execute (CalculateField)

Can anyone help me out?

  • 2
    There are missing parts in your question. Firstly what is variable n about which Python complains, and are the number of elements in temp_featureclasses and layers equal?
    – fatih_dur
    Aug 15, 2016 at 14:22
  • second @faith_dur the error specifically states "name 'n' is not defined" where have you used the unassigned variable 'n'
    – spk578
    Aug 15, 2016 at 14:28
  • I am unaware of any unassigned 'n' that I used, which is why I am fairly baffled by this error. I have posted my full script with previous parts. Am I not seeing something obvious?
    – mmoore
    Aug 15, 2016 at 14:33

1 Answer 1


The problem with your script is the expression. The expression must have the variable in single quotes since it is a string. There are a number of ways to format this. My favorite is as follows:

expression = """'{}'""".format(name)

Note the casting to string is not necessary since .format will do so automatically


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