title: tensorflow can be imported but it shows undefined when use it
body: <p>I installed tensorflow (GPU version) several days ago and it worked well. However, today I imported tensorflow as <code>import tensorflow as tf</code> and when I try to use it such as <code>tf.constant()</code>, it shows <code>NameError: name 'tf' is not defined</code>. I'm new to tensorflow. Anyone knows the reason?</p>
  
  <p>I found I have the same issue <a href="https://github.com/tensorflow/tensorflow/issues/8676" rel="nofollow noreferrer">https://github.com/tensorflow/tensorflow/issues/8676</a>
  I can run my script without any error message but there is no result.</p>