input: Write a function in Python to take a string input and reverse it. If the input is a single character, it should return the character itself. To test the function, we will use the following assert statements:
  
  assert reverse_string("hello") == "olleh"
  assert reverse_string("Python") == "nohtyP"
  assert reverse_string("12345") == "54321"
  assert reverse_string("a") == "a"
  assert reverse_string("") == ""
output: def reverse_string(s):
      if len(s) <= 1:
          return s
      return s[::-1]