code: def set_backend(name=None):
      
  
      possible = list(_BACKENDS)
      if name is None:
          names = []
      else:
          names = name.split(",")
  
      for name in reversed(names):
          for backend in list(possible):
              if backend.NAME == name:
                  possible.remove(backend)
                  possible.insert(0, backend)
                  break
          else:
              raise LookupError("Unkown backend: %r" % name)
  
      # only add null as fallback it explicitly specified
      if "null" not in names:
          possible = [b for b in possible if b.NAME != "null"]
  
      _ACTIVE_BACKENDS[:] = possible
docs: Set a prefered ffi backend (cffi, ctypes).
  
      set_backend() -- default
      set_backend("cffi") -- cffi first, others as fallback
      set_backend("ctypes") -- ctypes first, others as fallback