[Python] python adding new columns to dataframe

I am downloading some finance data and then adding new columns to a dataframe. But, when I saved the dataframe to an Excel file, the new columns I created are not getting saved in Excel.

import yfinance as yf
import pandas as pd
import time


stock_data = yf.download(tickers="AAPL", interval="1d", period="3y")

stock_data['SMA50'] = stock_data['Close'].rolling(window=50).mean()
stock_data['SMA200'] = stock_data['Close'].rolling(window=200).mean()
stock_data['buy?'] = ""

df = pd.DataFrame(stock_data)

for row in df.itertuples():
sma200 = row[7]
close = row[1]
if(sma200 > close):
print("buy!")
row[8] = "buy!"


writer = pd.ExcelWriter('AAPL.xlsx', engine='xlsxwriter')
df.to_excel(writer, sheet_name="datas", startrow=1, header=False, index=True)
workbook = writer.book
worksheet = writer.sheets["datas"]

writer.close()

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