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