code: def import_authors(self, tree):
          
          self.write_out(self.style.STEP('- Importing authors\n'))
  
          post_authors = set()
          for item in tree.findall('channel/item'):
              post_type = item.find('{%s}post_type' % WP_NS).text
              if post_type == 'post':
                  post_authors.add(item.find(
                      '{http://purl.org/dc/elements/1.1/}creator').text)
  
          self.write_out('> %i authors found.\n' % len(post_authors))
  
          authors = {}
          for post_author in post_authors:
              if self.default_author:
                  authors[post_author] = self.default_author
              else:
                  authors[post_author] = self.migrate_author(
                      post_author.replace(' ', '-'))
          return authors
docs: Retrieve all the authors used in posts
          and convert it to new or existing author and
          return the conversion.