input: Write a function to reverse an input string.
  
  def rev_string(s): 
     return s[::-1] 
    
  assert rev_string("Hello World!") == "!dlroW olleH"
  assert rev_string("Python") == "nohtyP"
  assert rev_string("") == ""
  assert rev_string("a") == "a"
  assert rev_string("12345") == "54321"
output: def rev_string(s): 
     return s[::-1]