Python检查回文的程序，为什么函数嵌套是错误的？

Question

# Program to check whether a string is a palindromes

# Prompts the user for strings and checks each one

# Exits when an empty string is entered

def isPalindrome(s):

    def toChars(s):

        """Converts a string to lowercase and removes non-letters

        Assumes s is a str.

        Converts uppercase letters to lowercase and removes non-letter"""

    # First of all, convert uppercase letters to lowercase

        s = s.lower()

    # Start with an empty string

        letterstring = ''

    # Go through s...

        for c in s:

            # ... and add the character to the string if it is a letter

            if c in 'abcdefghijklmnopqrstuvwxyz':

                letterstring += c

        return letterstring

    def isPal(s):

        """Checks whether the string s is a palindrome

        Assumes that s is a str with only lowercase letters and no non-letters.

        Returns True if s is a palindrome;

        Returns False otherwise.

        Recursive function."""

        if len(s) <= 1:

            # A palindrome of length 0 or 1 is a palindrome

            return True

        else:

            # Compare the first and the last letters and check the remainder

            # of the string

            return s[0] == s[-1] and isPal(s[1:-1])

    """Checks whether the string s is a palindrome

    Assumes s is a str.

    Returns True if the letters in s form a palindrome;

    Returns False otherwise.

    Case and non-letters are ignored."""

    return isPal(toChars(s))

str = input('Enter a string (empty string to exit): ')

while str != '':

    if isPalindrome(str):

        print(str, 'is a palindrome')

    else:

        print(str, 'is not a palindrome')

    str = input('Enter a string (empty string to exit): ')

print('Finished!')