docstr: transfer an index through a Unix pipe 
docstr_tok: ["transfer","an","index","through","a","Unix","pipe"]
code: def test_transfer_pipe(self):
      """transfer an index through a Unix pipe"""
  
      d, n = 32, 1000
      x = np.random.uniform(size=(n, d)).astype("float32")
      index = faiss.IndexFlatL2(d)
      index.add(x)
      Dref, Iref = index.search(x, 10)
  
      rf, wf = os.pipe()
  
      # start thread that will decompress the index
  
      def index_from_pipe():
          reader = faiss.PyCallbackIOReader(
              lambda size: os.read(rf, size)
          )
          return faiss.read_index(reader)
  
      with ThreadPool(1) as pool:
          fut = pool.apply_async(index_from_pipe, ())
  
          # write to pipe
          writer = faiss.PyCallbackIOWriter(lambda b: os.write(wf, b))
          faiss.write_index(index, writer)
  
          index2 = fut.get()
  
          # closing is not really useful but it does not hurt
          os.close(wf)
          os.close(rf)
  
      Dnew, Inew = index2.search(x, 10)
  
      np.testing.assert_array_equal(Iref, Inew)
      np.testing.assert_array_equal(Dref, Dnew)
  
imports: import numpy
  import unittest
  import faiss
  import tempfile
  import os
  import io
  import sys
  import pickle
  from multiprocessing.pool import ThreadPool
label_desc: Use image node_id 9 for calling the TestCallbacks obj's underlying member method code with example usage: obj.test_transfer_pipe() and returns: faiss