How to fill in data on CSV file if it already exisits?

I am trying to write a code that will allow for a user to input a code, once the code has been entered, the code will search for user on a csv file, if the user matches an existing entry on the csv file then it looks for the next available column in the users row and enters the number there. If there is no open slot then it will overwrite the first slot.

As you can see above, if user silverspoon enters a code of 8, then it should update the CODE3 column as it is empty. if goldspoon enters a code of 17, then it should update the CODE1 column since all columns have data. If bronzespoon enters a code of 99, then it should create a new row of data since they are not on the list. It should look like below.

This is a code I currently have but unfortunately I am making user enter all their codes all over again and it would updates the whole row every time new codes are entered.

    now = datetime.now()
    entrydate = now.strftime("%m/%d/%Y")
    fields = ["USER", "DATE", "CODE1", "CODE2", "CODE3", "CODE4", "CODE5"]
    df = pd.read_csv("C:\test\codes.csv")
    if username in df.values: # I use this to determine if the user is already on the CSV file
        with open("C:\test\codes.csv", "r") as csvfile, tempfile:
            reader = csv.DictReader(csvfile, fieldnames=fields)
            writer = csv.DictWriter(tempfile, lineterminator='\n', fieldnames=fields)
            for row in reader:
                if row["USER"] == username:
                    new_codes = [entrydate,code1, code2, code3, code4, code5]
                    row["DATE"], row["CODE1"], row["CODE2"], row["CODE3"], row["CODE4"], row["CODE5"] = new_codes
                row = {
                    "USER": row["USER"],
                    "DATE": row["DATE"],
                    "KEYWORD1": row["CODE1"],
                    "KEYWORD2": row["CODE2"],
                    "KEYWORD1": row["CODE3"],
                    "KEYWORD2": row["CODE4"],
                    "KEYWORD3": row["CODE5"],
                }
                writer.writerow(row)
        shutil.move(tempfile.name, "C:\test\codes.csv")
        print(f"{username} updated their code list of {code1} , {code2} , {code3} , {code4} , & {code5} effective {entrydate}")
        return
    else:
        with open("C:\test\codes.csv","a", newline="") as f:
            writer = csv.writer(f)
            writer.writerow([username,entrydate,code1,code2,code3,code4,code5])
            print(f"{username} added {code1} ,{code2} ,{code3} , {code4} , & {code5} to to their code list effective {entrydate}")
            return                

There is some error in your code, I modified it according to the rules you want ( when the user exist and no empty slot, when there is no empty slot and if the user dont exist ) dont forget to replace username and new_code with the user input.

import csv
import pandas as pd
import shutil
import os
from datetime import datetime
now = datetime.now()
entrydate = now.strftime("%m/%d/%Y")
fields = ["USER", "DATE", "CODE1", "CODE2", "CODE3", "CODE4", "CODE5"]
filename = "C:\\test\\codes.csv"
temp_filename = "C:\\test\\temp_codes.csv"
username = "silverspoon"
new_code = 8
df = pd.read_csv(filename)
if username in df.values:
with open(filename, "r") as csvfile, open(temp_filename, "w", newline="") as tempfile:
reader = csv.DictReader(csvfile, fieldnames=fields)
writer = csv.DictWriter(tempfile, lineterminator='\n', fieldnames=fields)
writer.writeheader()
for row in reader:
if row["USER"] == username:
for i in range(1, 6):
code_key = f"CODE{i}"
if not row[code_key]:
row[code_key] = new_code
break
else:
row["CODE1"] = new_code
row["DATE"] = entrydate
writer.writerow(row)
shutil.move(temp_filename, filename)
print(f"{username} updated their code list with {new_code} effective {entrydate}")
else:
with open(filename, "a", newline="") as f:
writer = csv.writer(f)
writer.writerow([username, entrydate, new_code, "", "", "", ""])
print(f"{username} added {new_code} to their code list effective {entrydate}")