input: 
  def is_palindrome(s: str) -> bool:
      """ Given a string s, check if it is a palindrome. 
      A palindrome is a string that reads the same forward and backward.
      
      >>> is_palindrome("racecar")
      True
      >>> is_palindrome("hello")
      False
      >>> is_palindrome("A man, a plan, a canal, Panama")
      True
      """
      s = ''.join(filter(str.isalnum, s)).lower()
      return s == s[::-1]
  
  print(is_palindrome("racecar"))
  
output:     
      s = ''.join(filter(str.isalnum, s)).lower()
      return s == s[::-1]
  