read from source file and write to target file line by line until the end of the source file. use a function to add the numbers in the source file and write to target file the sum.

def add_numbers_in_file(source_file, target_file):
    with open(source_file) as f:
        with open(target_file) as t:
            while True:
                line = f.readline()
                if not line:
                    break
                line_int = int(line)
                total += line_int
                t.write(line)
    f.close()
    t.close()
    return total

print(add_numbers_in_file("source.txt", "target.txt"))