replace strings in file

i = True
words = {"test","Hi","Hello"}
with open("yes.txt","r") as f:
  f = f.read()
  f = f.casefold()
for i in words:
  i = i.casefold()
  if i in f:
    f = f.replace(i,"@#$&*%")
with open("yes.txt","w") as d:
  d.write(f)