input: Write a python function to detect the current operating system and return it as a string.
  assert detect_os() in ['Mac OS', 'Linux OS']
  assert detect_os() != 'Windows OS'
output: def detect_os():
      import platform
      os_name = platform.system()
      if os_name == 'Darwin':
          return 'Mac OS'
      elif os_name == 'Linux':
          return 'Linux OS'
      else:
          return 'Unknown OS'