mit 6.0001 - recursive check for palindrome

pal_rec.py

def pal_rec(s):
  '''
  check if the alphabetical characters in a string read the same backwards and forewards
  '''
    # convert the input string to a list of lowercase alphanumeric characters only (i.e., punctuation, etc, omitted)
  def to_char(s):
    ls = list(s.lower())
    labc09 = list('abcdefghijklmnopqrstuvwxyz0123456789')
    char_s = [i for i in ls if i in labc09]
    return char_s

  # check if each character is a match with its front-back 'mirror index' character
  def pal_schar(char_s):
    if len(char_s) <= 1:
      return True
    else:
      return char_s[0] == char_s[-1] and pal_schar(char_s[1:-1])
  
  # return the Boolean result of the palindrome check applied to the (lowercase alphanumeric) string
  return(pal_schar(to_char(s)))

if __name__ == "__main__":
  s = input('Let\'s check for palindromes.\nWord or phrase:\n')
  if pal_rec(s):
    print(f'Yes, {s} is a palindrome')
  else:
    print(f'No, {s} is not a palindrome.')