Coderbyte Python Challenge: Simple Symbols

SimpleSymbols.py

# Have the function SimpleSymbols(str) take the str parameter being passed and
# determine if it is an acceptable sequence by either returning the string true or false.

# The str parameter will be composed of different symbols with several letters
# between them (ie. ++d+===+c++==a) and for the string to be true
# each letter must be surrounded by a + symbol.

# Examples:
# +t+a+b+ returns True
# +++a+s++++e+ returns True
# t+a+b+ returns False
# ta+b+s++++r+t returns False

def SimpleSymbols(str): 

    # import re to remove certain characters from string
    import re

    # create alphabet list
    alphabet = list('abcdefghijklmnopqrstuvwxyz')
    # convert string to lower
    str = str.lower()
    # set n = 0 for loop
    n = 0

    # create new var letters based on number of letters in string
    letter_str = re.sub("[^abcdefghijklmnopqrstuvwxyz]", "", str)
    letters = len(letter_str)

    # if first or last element in the string is in the alphabet then return False
    if str[0] in alphabet or str[-1] in alphabet:
        return False

    # otherwise, loop from second element to second to last element
    else:
        for x in range(1,len(str)-1):

            # get letter
            index = alphabet[x]

            # if letter in alphabet and the character before and after is a "+"
            # then add one to n, otherwise, dont add anything
            if index in alphabet and str[x-1] == "+" and str[x+1] == "+":
                n += 1
            else:
                n += 0

    # if n is equal to number of letters in string, then each letter in the string
    # has a "+" both before and after the letter
    if n == letters:
        return True
    else:
        return False
    
print(SimpleSymbols(input()))