how to find file using keywords python code example

Example 1: python read text file look for string

f = open(filename,  "r")
   whattoReturn = "None"
   if strToFind in f.read():
      whattoReturn = strToFind

Example 2: python search all txts in a folder

index_file =  open('index.txt', 'r')
    for line in index_file:
       ....