#!/usr/bin/env python3
import fileinput

with fileinput.FileInput(filename, inplace=True, backup='.bak') as f:
    for line in f:
        print(line.replace(text_to_search, replacement_text), end='')

#!/usr/bin/env python3

with open("test.txt", "a") as f:
    f.write("appended text")