title: Placeholders as fetches in Tensorflow
body: <p>I have seen Tensorflow code as following from a project:</p>
  
  <pre><code>sess.run(train_enqeue, feed_dict)
  </code></pre>
  
  <p>where train_enqeue is a built from:</p>
  
  <pre><code>train_queue = tf.FIFOQueue(train_params.async_encoding, [x.dtype for x in placeholders], name="train_queue")
  train_enqeue = train_queue.enqueue(placeholders)
  </code></pre>
  
  <p>It's basically a FIFO queue of placeholders. I wonder what does it mean to pass placeholders as fetches in this case? Does it return the values from the placeholder?  </p>
  
  <p>The code is from line 561 of <a href="https://github.com/allenai/document-qa/blob/master/docqa/trainer.py" rel="nofollow noreferrer">https://github.com/allenai/document-qa/blob/master/docqa/trainer.py</a> </p>