File modification (replace a line or lines of strings in a file)

import sys
f = open("yesterday2","r",encoding="utf-8")
f_new = open("yesterday2.bak","w",encoding="utf-8")

find_str = sys.argv [1 ]
replace_str = sys.argv[2]
for line in f:
    if find_str in line:
        line = line.replace(find_str,replace_str)
    f_new.write(line)
f.close()
f_new.close()