code: def _get_nb_pages(self):
          
          try:
              filelist = self.fs.listdir(self.path)
              count = 0
              for filepath in filelist:
                  filename = self.fs.basename(filepath)
                  if (filename[-4:].lower() != "." + ImgPage.EXT_IMG or
                      (filename[-10:].lower() == "." + ImgPage.EXT_THUMB) or
                      (filename[:len(ImgPage.FILE_PREFIX)].lower() !=
                          ImgPage.FILE_PREFIX)):
                      continue
                  count += 1
              return count
          except IOError as exc:
              logger.debug("Exception while trying to get the number of"
                           " pages of '%s': %s", self.docid, exc)
              return 0
          except OSError as exc:
              if exc.errno != errno.ENOENT:
                  logger.error("Exception while trying to get the number of"
                               " pages of '%s': %s", self.docid, exc)
                  raise
              return 0
docs: Compute the number of pages in the document. It basically counts
          how many JPG files there are in the document.