a.py

cesarkawakami@pandambp13r:~/temp/Asgaf [crap]
$ cat left_file.txt
relogio
poster
banana

cesarkawakami@pandambp13r:~/temp/Asgaf [crap]
$ cat right_file.txt
banana
colete
relogio

cesarkawakami@pandambp13r:~/temp/Asgaf [crap]
$ python x.py
poster

cesarkawakami@pandambp13r:~/temp/Asgaf [crap]
$ cat x.py
with open("right_file.txt", "r") as f:
    right_set = set(f.read().split())

with open("left_file.txt", "r") as f:
    for line in f:
        if line.strip() not in right_set:
            print line.strip()