topic Re: Syntax when passing variable to calculate field expression in Python Questions

Syntax when passing variable to calculate field expression

ZacharyHart — Wed, 14 Sep 2016 18:36:02 GMT

OK, so I feel like I've tried just about everything here. What I'm looking to do is to concatenate a iteration value (?) from a search cursor with the object ID of a feature class to create a unique ID field. I want these two values separated by a hyphen '-'. Target field (PLOT_ID) is obviously a string. There is what I've got working so far.

i = 1

ptCur = arcpy.da.SearchCursor (ptFC, fldLst)

for row in ptCur:

[various things happen removed for brevity]

Cluster_Plot = str(i) + '!OBJECTID!'

arcpy.AddMessage(Cluster_Plot)

arcpy.CalculateField_management(outPointsDissolve,"PLOT_ID",Cluster_Plot,"PYTHON_9.3")

things are appended to a a target FC and the target field looks like this:

So it appears to be treating the expression inputs as strings. From here I've tried to get a - in between the two values with no luck....even when converting the OID to a string : 'str(!OBJECTID!)' I just can't get things to escape right i guess. Things I've tried:

Cluster_Plot = str(i) + '-' + '!OBJECTID!'

Here the hyphen apparently is interpreted as an operator:

Cluster_Plot = str(i) + " + '-' + " + '!OBJECTID!'

Cluster_Plot = str(i) + " + '-' + " + 'str(!OBJECTID!)'

I dont get this because you can use the + operator in the field calculator just fine (if you format the field as a string using str)

Could someone please help me out of my misery here?

Re: Syntax when passing variable to calculate field expression

MicahBabinski — Sun, 12 Dec 2021 00:02:42 GMT

Hi Zachary,

Would

Cluster_Plot = '"{}-{}"'.format(i,'!OBJECTID!')‍

work? I've become a big fan of using triple quoted strings and the .format() method when creating a variable to use as a field calculator expression.

Another option is to use an update cursor. Something like:

table = "myTable"
fields = ("OBJECTID", "PLOT_ID")
i = 1
with arcpy.da.UpdateCursor(table, fields) as cursor:
    for row in cursor:
        row[1] = str(i) + "-" + str(row[0])
        cursor.updateRow(row)
        i+=1‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Good luck!

Re: Syntax when passing variable to calculate field expression

ZacharyHart — Wed, 14 Sep 2016 18:53:44 GMT

Hi Micah...I'm just getting a generic 'invalid syntax' error. for the first option.

The second wont work because the OIDs aren't coming from the FC used to run the loop/search cursor.

Re: Syntax when passing variable to calculate field expression

MicahBabinski — Wed, 14 Sep 2016 18:57:26 GMT

I see. Can you post your full script? Also, maybe try the field calculator expression without the str() around the i variable and !OBJECTID!.

Re: Syntax when passing variable to calculate field expression

ZacharyHart — Wed, 14 Sep 2016 20:03:30 GMT

I got it to work!!! Nice way to close out the day...for some reason the OID field needed single quotes too. If you want to modify your code I'll mark your answer as accepted.

Cluster_Plot = '"{}-{}"'.format(i,'!OBJECTID!')

Re: Syntax when passing variable to calculate field expression

MicahBabinski — Wed, 14 Sep 2016 20:07:12 GMT

Terrific! I've modified it. If at first you don't succeed...