Improvements?

[sparkz_alot]

You also do not need to specify 'str' in conjunction with the input statement, as 'input' by default, will create a string.

so this:

sentence = str(input("Enter a sentence without punctuation: ")).lower()

is the same as:

sentence = input("Enter a sentence without punctuation: ").lower()

You only need to prepend when you want something other than a string. For example, to expect an integer from the user:

answer = int(input("Enter an integer: ")