Using a dictionary to find unique values from one field, populate second field with consecutive integers [duplicate]

Question

I have an attribute table with three fields: Id, Name, and Id_2 and 18,701 records. I would like to populate Id_2 with consecutive integers for each unique value in the Name field. Prior to running the script, Id_2 is empty. Below is an example of what I would like the final attribute table to look like.

Below is the python script that I have developed. Note that I have built a dictionary using the Name field as the keys and the Id field as the values. I don't need to use the values, however, to accomplish this task, just the key. My code produces consecutive integers, however, the sequence doesn't reset with each unique value in the Name field. Rather, my code populates Id_2 with consecutive integers for the entire table, through record 18,701 (Not what I want!).

# import required python libraries or modules
import arcpy
import traceback

# settings
arcpy.env.workspace = r'D:/test'
arcpy.env.overwriteOutput = True

# variables
fc = r'D:/test/colors.shp'
fields = ['Id', 'Name', 'Id_2']
for f in fields:
  print f, f in [field.name for field in arcpy.ListFields(fc)]

# build dictionary
rnDict = {}
cursor = arcpy.da.SearchCursor(fc, fields)
try:
    for row in cursor:
        id = row[0]
        rn = row[1]
        rnDict.setdefault(rn, []).append(id)
    print 'dictionary successfully built'
except:
    print 'An error occurred - part 2'
    traceback.print_exc()
del cursor

# update Id_2 field with sequential integers
startNumber = 1
cursor = arcpy.da.UpdateCursor(fc, fields)  # type: object
try:
    for row in cursor2:
        name = row[1]
        if name in rnDict:
            row[2] = startNumber
            startNumber = startNumber + 1
            cursor2.updateRow(row)
    print 'completed consecutive numbering'
except:
    print 'An error occurred during calculating field'
    traceback.print_exc()
del cursor
# del rnDict

Answer 1

For a start ID is irrelevant in your fields, you only need the two fields being worked on 'Name', 'Id_2'. You've also set up a search then an update, you don't need to do this, a single update is enough; what you're after in much less code:

fc      = r'D:/test/colors.shp'
fields  = ['Name', 'Id_2']
NCntDic = {} # a new empty dictionary
with arcpy.da.UpdateCursor(fc,fields) as uCur:
    for uRow in uCur:
        ThisName = uRow[0].lower()    # this name as lower case
        if ThisName in NCntDic:
            NCntDic[ThisName]+= 1     # increment the counter
            uRow[1]=NCntDic[ThisName] # assign the value
        else:
            NCntDic[ThisName] = 1     # start the counter
            uRow[1]=NCntDic[ThisName] # assign the value
        uCur.updateRow(uRow)

The dictionary stores name:value pairs so each unique name (case sensitive) can keep a single count separate from the counts of other unique name. If your name:values do need to be case sensitive then take out the .lower() which forces all names to be lower case - also a good idea to .trim() if your data is suspect to remove leading and trailing spaces.