code: def list_backends():
      
      return ['ni'] + [name for (loader, name, ispkg) in pkgutil.iter_modules()
                       if name.startswith('pyvisa-') and not name.endswith('-script')]
docs: Return installed backends.
  
      Backends are installed python packages named pyvisa-<something> where <something>
      is the name of the backend.
  
      :rtype: list