Is there a better way to compare lists in arcpy?

Question

I simply want to create a script which compares a list with the field names of a featureclass and if the fields do not exist in the featureclass add them.

My code works, but is very inefficient. How can I compare lists in a more efficient manner?

import sys,os
import arcpy
from arcpy import env

env.overwriteOutput = True

workspace = r"PATH_TO_WORKSPACE"
fc = r"PATH_TO_FEATURECLASS"


lst_fcfields = arcpy.ListFields(fc)
lst_myfields = ["field1","field2"]

for field in lst_fcfields:
        for myfield in lst_myfields:
                if field.name ==myfield:
                        print fc +" - same as - " 
                else:
                        print field.name + " - not same as - " + myfield
                        arcpy.AddField_management(fc, myfield, "TEXT", "", "", 255)

Ideally, I would like the script to work interactively and ask the user if they would like to delete the fields which vary from the list, but as far as I know there are no dialogs in arcpy.

Answer 1

Do a set difference:

s.difference(t) s - t new set with elements in s but not in t

Ex:

l1 = ['apple','banana', 'celentro', 'donut', 'elephant', 'film', 'gopher','hyena',1,2,3,4,5]
l2 = ['film', 'celentro', 'badger', 'tiger', 100, 2, 4, 16]

>>> set(l1).difference(set(l2))
set([1, 3, 'apple', 'gopher', 'hyena', 'donut', 'elephant', 'banana', 5])

>>> set(l2).difference(set(l1))
set([16, 'tiger', 100, 'badger'])

Here is the performance difference, measured with cProfile:

>>> l1 = range(10000)
>>> l2 = range(5, 10005)
>>> cProfile.run('[i for i in l1 if not i in l2]')
         2 function calls in 1.472 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    1.472    1.472    1.472    1.472 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}


>>> cProfile.run('set(l1).difference(set(l2))')
          3 function calls in 0.009 seconds

    Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.003    0.003    0.009    0.009 <string>:1(<module>)
        1    0.006    0.006    0.006    0.006 {method 'difference' of 'set' objects}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}

For you you would do something like:

flds_to_add = set(lst_myfields).difference(set(lst_fcfields))
for field in flds_to_add:
  arcpy.AddField_management(fc, field, 'TEXT', '', '', 255)

Answer 2

This is a perfect use for list comprehension (two times!):

lst_fcfields = [f.name for f in arcpy.ListFields(fc)]
lst_myfields = ["field1","field2"]

## this line is the answer to the question       ----------------
diff_fields = [i for i in lst_myfields if not i in lst_fcfields]
## compare lists using an if within list comprehension ----------

for field in diff_fields:
    arcpy.AddField_management(fc, field, "TEXT", "", "", 255)

diff_fields can be read as: "every item in lst_myfields if that item is not in lst_fcfields"

For interactivity, as @Branco says, this would be easy if it were a stand-alone script. I don't have experience with tkinter, but in the simple console you could use something like:

for field in diff_fields:
    print field
    choice = raw_input("  add this field to the feature class? y/n ")
    if choice.lower().startswith("y"):
        arcpy.AddField_management(fc, field, "TEXT", "", "", 255)
        print "  field added"
    else:
        print "  field not added"

Answer 3

Maybe something like this.

import sys,os
import arcpy
from arcpy import env

env.overwriteOutput = True
workspace = r"PATH_TO_WORKSPACE"
fc = r"PATH_TO_FEATURECLASS"

lst_myfields = ["field1","field2"]

for field in [fieldfc.name for fieldfc in arcpy.ListFields(fc)]:
        if field in lst_myfields:
            print field +" - same as - "
        else:
            print field + " - not same"
            arcpy.AddField_management(fc, field, "TEXT", "", "", 255)