Popoulate fields based on occurrences and text

[CCWeedcontrol]

Trying to figure out how to populate two fields based on two fields. I need to be able to populate say F_ID3 with the number of occurrences of F_ID1 then in F_D4 add the text of the occurrences from F_ID2. Like below, note that their may be empty or nulls in the fields

Currently have this

OBJECTID	F_ID1	F_ID2
1	aa1123	12345
2	aa1123	12346
3	BB3361	67891
4	BB3361	67892
5	BB3361	67893
6	UU9832	11123
7	PP2225	6989
8	PP2225	6988

I need this

OBJECTID	F_ID1	F_ID2	F_ID3	F_ID4
1	aa1123	12345	2	12345, 12346
2	aa1123	12346
3	BB3361	67891	3	67891, 67892, 37893
4	BB3361	67892
5	BB3361	67893
6	UU9832	11123	1	11123
7	PP2225	6989	2	6988, 6989
8	PP2225	6988

[JoshuaBixby]

Quick and dirty, haven't tested, but may work for you:

import arcpy
from collections import defaultdict

lyr = 
fields = ["F_ID1", "F_ID2", "F_ID3", "F_ID4"]
sql = "ORDER BY F_ID1, F_ID2, OBJECTID"

d = defaultdict(list)
with arcpy.da.UpdateCursor(lyr, fields, sql_clause=(None,sql)) as cur:
    for id1, id2, id3, id4 in cur:
        d[id1] += id2
    
    cur.reset()
    id1, id2, id3, id4 = next(cur)
    cur.updateRow([id1, id2, len(d[id1]), ",".join(d[id1])])
    id_prev = id1
    for id1, id2, id3, id4 in cur:
        if id1 == id_prev:  continue
        cur.updateRow([id1, id2, len(d[id1]), ",".join(d[id1])])
        id_prev = id1
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[2Quiker]

I wasn't getting what I needed from the code you provided but I was able to make the following work based on the code you provided. It's not as clean as yours but I am a newbie to python. Thanks.

#populate F_ID3 & F_ID4
dict1 = dict()
with arcpy.da.SearchCursor(fc,['F_ID1','F_ID3','F_ID2']) as cursor:
    for row in cursor:
        dict1.setdefault(row[0],[]).append(str(row[2]))

with arcpy.da.UpdateCursor(fc,['F_ID1','F_ID3','F_ID4']) as cursor:
    for row in cursor:
        row[2] = ",".join(dict1[row[0]])
        cursor.updateRow(row)

#Populate F_ID3    
with arcpy.da.UpdateCursor(fc, ["F_ID1", "F_ID3"]) as cur:
    cnt = Counter(row[0] for row in cur)
    cur.reset()
    for row in cur:
        cur.updateRow([row[0], cnt.pop(row[0], None)])‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍