how replace words with number in fillna in python code example

Example 1: python pandas replace nan with null

df.fillna('', inplace=True)

Example 2: fillna string

# importing pandas module 
import pandas as pd 
  
# making data frame from csv file 
nba = pd.read_csv("nba.csv") 
  
# replacing na values in college with No college 
nba["College"].fillna( method ='ffill', inplace = True)