trouble with code block execution in python script

Question

I am trying to calculate values for a field in a python script, but keep getting an error that I don't understand:

ExecuteError: ERROR 000539: Error running expression: Fron000000F1 
Traceback (most recent call last):
  File "<expression>", line 1, in <module>
NameError: name 'Fron000000F1' is not defined
Failed to execute (CalculateField).

I can't figure out where the name 'Fron000000F1' is coming from to try and track this problem. I have tested the expression in arcmap using the field calculator with no problems. What is wrong? Code is below:

import arcpy, os, csv
arcpy.env.overwriteOutput = True

out_fc_pm = r'C:\temp\temp.gdb\SignDetect__SOL680_DD_with_PM'

def calc_id(pm,pict_fld):
  import re
  pm_patt = re.compile('\d*\.*\d*')
  pm_match = re.findall(pm_patt,pm[5:])[0]
  pm = pm_match.replace('.','')
  pm_final = pm.zfill(6)
  dir_patt = re.compile('[A-Z]\d*')
  dir_match = re.findall(dir_patt,pict_fld)[0]
  id = pict_fld[:5]+pm_final+dir_match
  if id[-1:].isdigit():
    return id
  else:
    return id+'1'

arcpy.AddField_management(out_fc_pm,'OSMI_ID','TEXT')
arcpy.CalculateField_management(out_fc_pm,'OSMI_ID',calc_id('!POSTMILE!','!Front!'),'PYTHON_9.3')

Answer 1

When using Field Calculator in ArcPy, the code block needs to be defined within the arcpy.CalculateField_management function.

Syntax:

CalculateField_management (in_table, field, expression, {expression_type}, {code_block})

So,

arcpy.CalculateField_management(out_fc_pm,'OSMI_ID',"calc_id('!POSTMILE!','!Front!')",'PYTHON_9.3',[yourcodeblock])

where [yourcodeblock] goes all on one line, and I don't want to risk mis-copying it. Per this Answer, you should be able to get the correct syntax for your script using:

run Calculate Field tool once interactively using the Expression and Code Block ... and then Copy As Python Snippet in the Geoprocessing | Results window

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Side note: A complex expression seems well suited to an UpdateCursor instead of Field Calculator. (It would certainly be easier to read in your code.)