code: def _start_browsing_some_sites(self):
          '''
          Starts browsing some sites.
  
          Raises:
              NoBrowsersAvailable if none available
          '''
          # acquire_multi() raises NoBrowsersAvailable if none available
          browsers = self._browser_pool.acquire_multi(
                  (self._browser_pool.num_available() + 1) // 2)
          try:
              sites = self._frontier.claim_sites(len(browsers))
          except:
              self._browser_pool.release_all(browsers)
              raise
  
          for i in range(len(browsers)):
              if i < len(sites):
                  th = threading.Thread(
                          target=self._brozzle_site_thread_target,
                          args=(browsers[i], sites[i]),
                          name="BrozzlingThread:%s" % browsers[i].chrome.port,
                          daemon=True)
                  with self._browsing_threads_lock:
                      self._browsing_threads.add(th)
                  th.start()
              else:
                  self._browser_pool.release(browsers[i]f _start_browsing_some_sites(self):
          '''
          Starts browsing some sites.
  
          Raises:
              NoBrowsersAvailable if none available
          '''
          # acquire_multi() raises NoBrowsersAvailable if none available
          browsers = self._browser_pool.acquire_multi(
                  (self._browser_pool.num_available() + 1) // 2)
          try:
              sites = self._frontier.claim_sites(len(browsers))
          except:
              self._browser_pool.release_all(browsers)
              raise
  
          for i in range(len(browsers)):
              if i < len(sites):
                  th = threading.Thread(
                          target=self._brozzle_site_thread_target,
                          args=(browsers[i], sites[i]),
                          name="BrozzlingThread:%s" % browsers[i].chrome.port,
                          daemon=True)
                  with self._browsing_threads_lock:
                      self._browsing_threads.add(th)
                  th.start()
              else:
                  self._browser_pool.release(browsers[i])
docs: Starts browsing some sites.
  
          Raises:
              NoBrowsersAvailable if none available