input: 
  def is_palindrome(s: str) -> bool:
      """Check if the string is a palindrome.
      >>> is_palindrome("racecar")
      True
      >>> is_palindrome("hello")
      False
      >>> is_palindrome("a")
      True
      >>> is_palindrome("A man, a plan, a canal, Panama")
      True
      """
      s = ''.join(e for e in s if e.isalnum()).lower()
      return s == s[::-1]
  
  print(is_palindrome("racecar"))
  
output:     s = ''.join(e for e in s if e.isalnum()).lower()
      return s == s[::-1]