input: Write a function to reverse a given string. If the given string is empty, return an empty string. 
  
  def reverse_string(input_string):
      return input_string[::-1]
  
  assert reverse_string("Python") == "nohtyP"
  assert reverse_string("Hello") == "olleH"
  assert reverse_string("") == ""
  assert reverse_string("12345") == "54321"
output: def reverse_string(input_string):
      return input_string[::-1]