Auto Incrementing Field based on Selection of Records in Feature Class?

Question

I'm attempting to write a script that will add an incremented value to a field based on a selection of records.

For example, the script will select the first 300 records in a feature class and then using calculate field assign a value of 1 to a field called "Week_Assignment". It will then select the next 300 records and assign a value of 2 to the same field, etc.

I'm able to select the records, but not able to calculate the field with an incremented value. I found some generic code on an Esri help forum that will auto increment every row in a field, but I need to modify it to add the same value to all rows in a selection.

Here's the sample code:

rec=0 
def autoIncrement(): 
 global rec 
 pStart = 1  
 pInterval = 1 
 if (rec == 0):  
  rec = pStart  
 else:  
  rec += pInterval  
 return rec

autoIncrement()

Answer 1

I modified the sample code provided to account for a user-specified group interval. This uses division to identify the group number. I add pStart to account for the groups starting with 1.

rec=0 
def autoIncrement(groupinterval): 
 global rec
 pStart = 1  
 pInterval = 1 
 if (rec == 0):  
  pGroup = pStart
  rec = pStart 
 else:
  pGroup = int(rec/groupinterval) + pStart #check with your version of Python; should be floor operation for integer division. 
  rec += pInterval  
 return pGroup

autoIncrement(300)

Since autoincrement(300) is a string parameter for the CalculateField geoprocessing tool, you could modify to use a variable in place here and then build the string, e.g.

usrval = 300 
increxp = "autoincrement({})".format(usrval)
arcpy.CalculateField_management(week_lyr, "Week_Assignment", increxp, "PYTHON", "rec=0 \\ndef autoIncrement(groupinterval): \\n global rec\\n pStart = 1 \\n pInterval = 1 \\n if (rec == 0): \\n pGroup = pStart\\n rec = pStart \\n else:\\n pGroup = int(rec/groupinterval) + pStart \\n rec += pInterval \\n return pGroup")