input: Write a function to reverse an input string. If the string is empty, return an empty string as well.
  def rev_string(s): 
     return s[::-1] 
     
  assert rev_string("Hello World!") == "!dlroW olleH"
  assert rev_string("Python") == "nohtyP"
  assert rev_string("") == ""
output: def rev_string(s): 
     return s[::-1]