code: def backend():
      
  
      if _module_values['backend'] is not None:
          return _module_values['backend']
  
      with _backend_lock:
          if _module_values['backend'] is not None:
              return _module_values['backend']
  
          if sys.platform == 'win32':
              # Windows XP was major version 5, Vista was 6
              if sys.getwindowsversion()[0] < 6:
                  _module_values['backend'] = 'winlegacy'
              else:
                  _module_values['backend'] = 'win'
          elif sys.platform == 'darwin':
              _module_values['backend'] = 'osx'
          else:
              _module_values['backend'] = 'openssl'
  
          return _module_values['backend']
docs: :return:
          A unicode string of the backend being used: "openssl", "osx", "win",
          "winlegacy"