concatenate where one field may be null

Question

I am trying to run this script and where I am conactenating a first name and a lastname with a space inbetween. What is happening is that if the first name is null I get a blank space in front for the owner name. I am using a file geodatabase.
I have left out the other code that is not needed for this exaple.

Here is my code that runs w/o error but puts a space in front for null firstname.

RPT_PARCELS_OWNER_DATA_with_OwnerName_Field = "C:\Application Database\RPTSA.gdb\RPT_PARCELS_OWNER_DATA"

gp.CalculateField_management(RPT_PARCELS_OWNER_DATA_with_OwnerName_Field, "OwnerName", "[FIRST_NAME] +" "+ [LAST_NAME]", "VB", "")

I have tried an expression and a codeblock but I get error message.

Here is my try:

expression = 'concat(!FIRST_NAME!,!LAST_NAME!)'
codeBlock = \
    "def concat(first,last):\
    if first == '':\
    return !LAST_NAME!\
    else:\
    return !FIRST_NAME! + ' ' + !LAST_NAME!"

gp.CalculateField_management(RPT_PARCELS_OWNER_DATA_with_OwnerName_Field, "OwnerName", expression, "PYTHON", codeBlock)
  
The error message says the parameters are not valid.
Thanks for help, I am a novice at python.

PeterHoffman · Answer

I sent a interim version, I had changed the variables as you suggested.

Anyhow, I found some better examples and came up with this which works:

expression = "concatname(str(!FIRST_NAME!),str(!LAST_NAME!))"
def concatname(first,last):
 first = first.strip()
 retval = first + ' ' + last
 return retval.strip()

gp.CalculateField_management(RPT_PARCELS_OWNER_DATA_with_OwnerName_Field, "OwnerName", expression, "PYTHON")

DarrenWiens2 · Answer

"def concat(first,last):\
 if first == '':\
 return !LAST_NAME!\
 else:\
 return !FIRST_NAME! + ' ' + !LAST_NAME!"

You've passed the values of FIRST_NAME and LAST_NAME into variables called first and last. Also, I'm pretty sure you need to maintain proper Python indentation. So:"def concat(first,last):\
    if first == '':\
        return last\
    else:\
        return first + ' ' + last"