arcgis desktop - How calculate sequential values based on group field?

In field calculator i calculate sequential values in field [ORDER_ID] What need change in script to calculate sequential values based on group field [GROUP_ID]. I select values manually and use autoincrement () in Field Calculator.

Parser: Python Expression: autoIncrement() Pre-Logic Script Code:

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

This is the result. Field ORDER_ID is already calculated manually (i select GROUP_ID and use autoincrement).

What changes script needed to use it in field calculator?

Answer

You can do this with an UpdateCursor easily in the Python Window:

import arcpy

vals = []
fields = ['ORDER_ID', 'GROUP_ID'] #your field names

with arcpy.da.UpdateCursor(your_lyr, fields) as rows:
    for r in rows:
        vals.append(r[1])
        r[0] = vals.count(r[1])
        rows.updateRow(r)

where the your_lyr variable is a reference to the table you're updating.