Can I accomplish this in an easier more fluid way? (in ArcPy)

Question

For a project I am adding fields and then populating those fields with data already contained in the table. The adding fields is easy.

arcpy.AddField_management("PLSSFirstDivision","TRS","TEXT","","",20) 
arcpy.AddField_management("PLSSFirstDivision","TWN","TEXT","","",20)
arcpy.AddField_management("PLSSFirstDivision","SEC","TEXT","","",20)
arcpy.AddField_management("PLSSFirstDivision","RNG","TEXT","","",20) 
arcpy.AddField_management("PLSSFirstDivision","TWN_D","TEXT","","",20)
arcpy.AddField_management("PLSSFirstDivision","RNG_D","TEXT","","",20) 

Then I need to take specific number from a field (string) and I could only get it to work in ArcMaps Calculator and not the Python window. The data looked like this: (needed bold)

LA180230N0120E0SN100

TWN = MID([FRSTDIVID],6,2)
RNG = MID([FRSTDIVID],11,2)
SEC = MID([FRSTDIVID],18,2)

Then I needed to strip the initial "0" for those 3 fields:

TWN = !TWN!.lstrip('0')
RNG = !RNG!.lstrip('0')
SEC = !SEC!.lstrip('0')

Than adding it all together in a final field:

TRS = "T"+ [TWN]+ [TWN_D]+"R" + [RNG]+ [RNG_D]+"-" + "SEC" + [SEC]

Thanks for any help, just trying to learn more

Answer 1

I haven't actually run these, so my syntax could be off a tad, but you need to come up with a python expression that results in the strings you want, then use arcpy's CalculateField method to update your table. If you test your expressions in the field calculator window, you should be able to just copy/paste your final expression into the statements like below.

arcpy.CalculateField_management("PLSSFirstDivision", "TWN", "!FRSTDIVID![6:2].lstrip('0')", "PYTHON3")
arcpy.CalculateField_management("PLSSFirstDivision", "TRS", "'T' + !TWN!+ !TWN_D!+'R' + !RNG!+ !RNG_D!+'-SEC' + !SEC!", "PYTHON3")

Answer 2

These are the sort of complex attribute manipulations that I like to use an UpdateCursor for. You can manipulate the contents of multiple fields in one iteration, and then write the updates out for each row at once.

with arcpy.da.UpdateCursor("PLSSFirstDivision", ["FRSTDIVID","TRS","TWN","SEC","RNG","TWN_D","RNG_D"])  as cursor:
    for row in cursor:
        frstdivid = row[0]
        # try string slicing for this instead of the `MID` function
        # and you can strip leading zeroes in the same line
        twn = frstdivid[5:7].lstrip('0')
        rng = frstdivid[10:12].lstrip('0')
        sec = frstdivid[17:19].lstrip('0')
        # was not sure how twn_d and rng_d are calculated based on your provided code, but...
        twn_d = foo
        rng_d = bar
        # use all these to calculate trs
        trs = 'T{}{}R{}{}-{}SEC'.format(twn, twn_d, rng, rng_d, sec)
        # assign the calculated values back to row positions
        row[1] = trs
        row[2] = twn
        row[3] = sec
        row[4] = rng
        row[5] = twn_d
        row[6] = rng_d
        # write the new row with complete values from memory to your table
        cursor.updateRow(row)