title: TensorFlow Error found in Tutorial
body: <p>Dare I even ask? This is such a new technology at this point that I can't find a way to solve this seemingly simple error. The tutorial I'm going over can be found here- <a href="http://www.tensorflow.org/tutorials/mnist/pros/index.html#deep-mnist-for-experts">http://www.tensorflow.org/tutorials/mnist/pros/index.html#deep-mnist-for-experts</a></p>
  
  <p>I literally copied and pasted all of the code into IPython Notebook and at the very last chunk of code I get an error.</p>
  
  <pre><code># To train and evaluate it we will use code that is nearly identical to that for the simple one layer SoftMax network above.
  # The differences are that: we will replace the steepest gradient descent optimizer with the more sophisticated ADAM optimizer.
  
  cross_entropy = -tf.reduce_sum(y_*tf.log(y_conv))
  train_step = tf.train.AdamOptimizer(1e-4).minimize(cross_entropy)
  correct_prediction = tf.equal(tf.argmax(y_conv,1), tf.argmax(y_,1))
  accuracy = tf.reduce_mean(tf.cast(correct_prediction, "float"))
  sess.run(tf.initialize_all_variables())
  for i in range(20000):
      batch = mnist.train.next_batch(50)
      if i%100 == 0:
          train_accuracy = accuracy.eval(feed_dict={x:batch[0], y_: batch[1], keep_prob: 1.0})
      print "step %d, training accuracy %g"%(i, train_accuracy)
      train_step.run(feed_dict={x: batch[0], y_: batch[1], keep_prob: 0.5})
  
  print "test accuracy %g"%accuracy.eval(feed_dict={
      x: mnist.test.images, y_: mnist.test.labels, keep_prob: 1.0})
  </code></pre>
  
  <p>After running this code, I receive this error.</p>
  
  <pre><code>---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-46-a5d1ab5c0ca8&gt; in &lt;module&gt;()
       15 
       16 print "test accuracy %g"%accuracy.eval(feed_dict={
  ---&gt; 17     x: mnist.test.images, y_: mnist.test.labels, keep_prob: 1.0})
  
  /root/anaconda/lib/python2.7/site-packages/tensorflow/python/framework/ops.pyc in eval(self, feed_dict, session)
      403 
      404     """
  --&gt; 405     return _eval_using_default_session(self, feed_dict, self.graph, session)
      406 
      407 
  
  /root/anaconda/lib/python2.7/site-packages/tensorflow/python/framework/ops.pyc in _eval_using_default_session(tensors, feed_dict, graph, session)
     2712     session = get_default_session()
     2713     if session is None:
  -&gt; 2714       raise ValueError("Cannot evaluate tensor using eval(): No default "
     2715                        "session is registered. Use 'with "
     2716                        "DefaultSession(sess)' or pass an explicit session to "
  
  ValueError: Cannot evaluate tensor using eval(): No default session is registered. Use 'with DefaultSession(sess)' or pass an explicit session to eval(session=sess)
  </code></pre>
  
  <p>I thought that I may need to install or reinstall TensorFlow via conda install <a href="https://storage.googleapis.com/tensorflow/linux/cpu/tensorflow-0.5.0-cp27-none-linux_x86_64.whl">https://storage.googleapis.com/tensorflow/linux/cpu/tensorflow-0.5.0-cp27-none-linux_x86_64.whl</a> but conda doesn't even know how to install it.</p>
  
  <p>Does anyone have any idea of how to work around this error?</p>