text: # -*- coding: utf-8 -*-
  # Natural Language Toolkit: JSON Encoder/Decoder Helpers
  #
  # Copyright (C) 2001-2015 NLTK Project
  # Author: Steven Xu <xxu@student.unimelb.edu.au>
  #
  # URL: <http://nltk.org/>
  # For license information, see LICENSE.TXT
  
  """
  Register JSON tags, so the nltk data loader knows what module and class to look for.
  
  NLTK uses simple '!' tags to mark the types of objects, but the fully-qualified
  "tag:nltk.org,2011:" prefix is also accepted in case anyone ends up
  using it.
  """
  
  import json
  
  json_tags = {}
  
  TAG_PREFIX = '!'
  
  def register_tag(cls):
      """
      Decorates a class to register it's json tag.
      """
      json_tags[TAG_PREFIX+getattr(cls, 'json_tag')] = cls
      return cls
  
  class JSONTaggedEncoder(json.JSONEncoder):
      def default(self, obj):
          obj_tag = getattr(obj, 'json_tag', None)
          if obj_tag is None:
              return super(JSONTaggedEncoder, self).default(obj)
          obj_tag = TAG_PREFIX + obj_tag
          obj = obj.encode_json_obj()
          return {obj_tag: obj}
  
  class JSONTaggedDecoder(json.JSONDecoder):
      def decode(self, s):
          return self.decode_obj(super(JSONTaggedDecoder, self).decode(s))
  
      @classmethod
      def decode_obj(cls, obj):
          # Decode nested objects first.
          if isinstance(obj, dict):
              obj = dict((key, cls.decode_obj(val)) for (key, val) in obj.items())
          elif isinstance(obj, list):
              obj = list(cls.decode_obj(val) for val in obj)
          # Check if we have a tagged object.
          if not isinstance(obj, dict) or len(obj) != 1:
              return obj
          obj_tag = next(iter(obj.keys()))
          if not obj_tag.startswith('!'):
              return obj
          if obj_tag not in json_tags:
              raise ValueError('Unknown tag', obj_tag)
          obj_cls = json_tags[obj_tag]
          return obj_cls.decode_json_obj(obj[obj_tag])
  
  __all__ = ['register_tag', 'json_tags',
             'JSONTaggedEncoder', 'JSONTaggedDecoder']
  