As Georg says: you shouldn't be mixing Unicode objects and string
objects. It's perfectly valid for string.letters to contain non-ASCII
bytes, and it's no surprise that this fails for you. string.letters
indeed *does* contain only letters.

In any case, testing for letter-ness by using "in string.letters" is not
a good idea, as it involves a linear search. I recommend to use

u"a".isalpha() 

instead