text: from __future__ import absolute_import, division, unicode_literals
  
  
  from html5lib import treewalkers
  
  from .htmlserializer import HTMLSerializer
  from .xhtmlserializer import XHTMLSerializer
  
  def serialize(input, tree="simpletree", format="html", encoding=None,
                **serializer_opts):
      # XXX: Should we cache this?
      walker = treewalkers.getTreeWalker(tree) 
      if format == "html":
          s = HTMLSerializer(**serializer_opts)
      elif format == "xhtml":
          s = XHTMLSerializer(**serializer_opts)
      else:
          raise ValueError("type must be either html or xhtml")
      return s.render(walker(input), encoding)
  