input: Write a function to reverse an input string using Python. The function should take a string as input and return the reversed string.
  def rev_string(s): 
     return s[::-1]
  
  assert rev_string("Hello World!")=="!dlroW olleH"
  assert rev_string("abc")=="cba"
  assert rev_string("")==""
  assert rev_string("12345")=="54321"
  assert rev_string("racecar")=="racecar"
output: def rev_string(s): 
     return s[::-1]