title: TensorFlow: TypeError: Using a `tf.Tensor` as a Python `bool` is not allowed
body: <p>I'm trying to define a triplet loss using descriptor from a CNN's output, but this error showed up when I try to train the network.</p>
  
  <p>My definition of loss function:</p>
  
  <pre><code>def compute_loss(descriptor, margin):
      diff_pos = descriptor[0:1800:3] - descriptor[1:1800:3]
      diff_neg = descriptor[0:1800:3] - descriptor[2:1800:3]
      Ltriplet = np.maximum(0, 1 - tf.square(diff_neg)/(tf.square(diff_pos) + margin))
      Lpair = tf.square(diff_pos)
  
      Loss = Ltriplet + Lpair
  
      return Loss
  </code></pre>
  
  <p>here descriptor is the outcome of CNN, the income of CNN is a set of triplets containing anchor, puller and pusher exactly in this order. As input I packed 600 triplet together and feed them into the CNN. </p>
  
  <p>Then I got this error when training the network:</p>
  
  <pre><code>2018-03-08 16:40:49.529263: I tensorflow/core/platform/cpu_feature_guard.cc:140] Your CPU supports instructions that this TensorFlow binary was not compiled to use: AVX2 FMA
  Traceback (most recent call last):
    File "/Users/gaoyingqiang/Documents/GitHub/Master-TUM/TDCV/exercise_3/ex3/task2_new.py", line 78, in &lt;module&gt;
      loss = compute_loss(h_fc2, margin)
    File "/Users/gaoyingqiang/Documents/GitHub/Master-TUM/TDCV/exercise_3/ex3/task2_new.py", line 37, in compute_loss
      Ltriplet = np.maximum(0, 1 - tf.square(diff_neg)/(tf.square(diff_pos) + margin))
    File "/Users/gaoyingqiang/.virtualenvs/ex3/lib/python3.6/site-packages/tensorflow/python/framework/ops.py", line 614, in __bool__
      raise TypeError("Using a `tf.Tensor` as a Python `bool` is not allowed. "
  TypeError: Using a `tf.Tensor` as a Python `bool` is not allowed. Use `if t is not None:` instead of `if t:` to test if a tensor is defined, and use TensorFlow ops such as tf.cond to execute subgraphs conditioned on the value of a tensor.
  
  Process finished with exit code 1
  </code></pre>
  
  <p>Where goes wrong?</p>