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