input: Write a function to reverse the characters of a string. The function should take a string argument and return a new string with the characters in reverse order.
  
  def reverse_string(input_string):
      return input_string[::-1]
  
  assert reverse_string("hello")=="olleh"
  assert reverse_string("world")=="dlrow"
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("")==""
  assert reverse_string("123")=="321"
output: def reverse_string(input_string):
      return input_string[::-1]