How to create a new column from a constant value in a DateFrame

The following code below should work. You just need to assign the RACENo as str (not pandas.Series) to the new column. I.e. there is no need to convert RACENo to a pandas.Series.

There are some ways to do it:

1. df['RaceNo'] = RACENo - Insert/replace a column with the given value to all rows;
2. df = df.assign(RaceNo=RACENo) - assign a new column to the DataFrame
3. df.insert(0, 'RaceNo', RACENo) - Insert a column at a specified position
4. df.loc[:, 'RaceNo'] = RACENo - Create a new column and assign the new value

import time
from numpy import NaN
import pandas as pd
from selenium import webdriver
from selenium.webdriver.chrome.service import Service
from bs4 import BeautifulSoup

s = Service(executable_path=r'D:\Python3104\chromedriver.exe')
driver = webdriver.Chrome(service=s)
driver.maximize_window()
url = '''http://racing.hkjc.com/racing/information/Chinese/Reports/CORunning.aspx?
Date=20220701&RaceNo=2'''
driver.get(url)
time.sleep(10)

#Got the RaceNo from URL
soup = BeautifulSoup(driver.page_source, "html.parser")
RACENo = ((driver.current_url.split("RaceNo=")[1]))

#Get a DataFrame From HTML
df = pd.read_html(
str(soup.find("table", class_="table_bd f_fs13")))[0]

#Add a column to DataFrame using RACEno
df['RaceNo'] = RACENo

To insert a new column to the existing dataframe you need to assign the values of your Series in that way:

df['RaceNo'] = RACENo.values