ArcGIS keeps throwing an error when I use the following script in the field calculator, using Python:

pre-logic script code:

y= ()
if !fieldx!.isalnum():
    y= 1
    y= 0



The error in the results window reads:

'ERROR 999999: Error executing function, Syntax Error, Failed to execture (CalculateField)'

I've checked the classes of the fields: fieldx is a string and fieldy a short integer, so that can't be the problem. I've also enabled the shapefile for editing. Note that I've added fieldy using the addfield function.

Other fieldcalculations, for example:

fieldy = 5

do work, so the problem should be in the syntax.

What am I doing wrong?

  • 2
    Is the !fieldx! notation something specific to arcgis? In standard Python, that would be the syntax error.
    – Thomas K
    Jan 26, 2012 at 12:41
  • Hi Thomas, thanks for your consideration. Yes, I believe that's specific for ArcGIS. When I insert the fieldname by clicking the field from the field list, this is how it appears in the code. ArcGIS help docs also use this notation in their Python examples.
    – HDR
    Jan 26, 2012 at 12:47

1 Answer 1


That needs to be treated as a function.

In your code block:

def AlnumField(field_value):
   if str(field_value).isalnum():
     return 1
     return 0

Then in your expression: AlnumField(!fieldx!)

The !fieldname! notation is only respected in the expression, not the code block.

  • In python the .isalnum() method is a string method. I'm not sure this will work if an int is provided as the field_value.
    – garnertb
    Jan 27, 2012 at 20:11
  • Fixed, will now handle ints and friends. Feb 28, 2012 at 8:41

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