# Avoid this

lyrics_list = ['her', 'name', 'is', 'rio']

words = make_wordlist() # Pretend this returns many words that we want to test

for word in words:

    if word in lyrics_list: # Linear time

        print word, "is in the lyrics"

# Do this

lyrics_list = ['her', 'name', 'is', 'rio']

lyrics_set = set(lyrics_list) # Linear time set construction

words = make_wordlist() # Pretend this returns many words that we want to test

for word in words:

    if word in lyrics_list: # Constant time

        print word, "is in the lyrics"

the second example should read ... if word in lyrics_set: ...