Python dictionaries subclassification

I am using ElasticSearch in my Python application and want to be able to create a reusable dictionary object that represents the query. JSON structures are described here http://pulkitsinghal.blogspot.co.uk/2012/02/how-to-use-elasticsearch-query-dsl.html and I use PyES to query the search server. With PyES, we can pass a dict object that gets jsonified before sending it to the server. I want to create a library of general queries in which only the actual term of the request changes, so I thought that I would subclass the dict so that I could, for example, pass the request to the term through the constructor, and when the dict gets jsonified, I end up with something like of this:

{
  "fields": [
    "name",
    "shortDescription",
    "longDescription"
  ],
  "query": {
    "query_string": {
      "fields": [
        "name"
      ],
      "query": query_term,
      "use_dis_max": true
    }
  }
}

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:

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class StandardQuery(object):

      search_fields = ['meta_keywords', 'meta_description', \
                       'fields.title.value', 'slug']
      return_fields = ['slug', 'fields.title.value']

      def __init__(self, query):

          self.query = query

      def to_dict(self):

          output = dict()

          output['fields'] = self.search_fields
          output['query'] = {'query_string': {'fields': self.return_fields, \
                                              'query': self.query, 'use_dis_max': True}}

          return output