Python 2.7 : TypeError: 'builtin_function_or_method' object is not subscriptable

I'm trying to make Scrabble in Python. The rack (where the 7 letters are) is a list where I appended 7 times tk.StringVar() Every time the player want to drop a word, I have to get the letters off the rack. Here's my problem. As I can associate the letters of the word dropped and the letters of the rack, I created a provisional list where I append each StringVar.get() from the initial rack. Then I created a code that change the dropped letters by an empty string '' as I can associate the initial rack with the provisional list. Here's my code ``` def defausse_rack_prov(word,rack_prov): word=word.get() for i in word: if i.upper() in rack_prov: rack_prov[rack_prov.index[i]]='' #This is the problematic line return rack_prov ``` I keep getting ``` TypeError: 'builtin_function_or_method' object is not subscriptable ``` What should I do ? Thank you :)