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