title: how can I get my Python 3.6 back after installing Tornado 4.5.3?
body: <p>I use <code>miniconda</code> to run <code>Python 3.6</code>. Everything was fine until I couldn't start Jupyter notebook. The error message was:</p>
  
  <pre><code>File "/Users/usr/miniconda3/bin/jupyter-notebook", line 7, in &lt;module&gt;
  from notebook.notebookapp import main
  
  
  File "/Users/usr/miniconda3/lib/python2.7/site-packages/notebook/notebookapp.py", line 45, in &lt;module&gt;
      ioloop.install()
    File "/Users/usr/miniconda3/lib/python2.7/site-packages/zmq/eventloop/ioloop.py", line 210, in install
      assert (not ioloop.IOLoop.initialized()) or \
  AttributeError: type object 'IOLoop' has no attribute 'initialized'
  </code></pre>
  
  <p>After a Google search, I tried the following:<br>
  <code>conda install tornado=4.5.3</code>
  But it gave a warning message: </p>
  
  <pre><code>CryptographyDeprecationWarning: Support for your Python version is deprecated. The next version of cryptography will remove support. Please upgrade to a 2.7.x release that supports hmac.compare_digest as soon as possible.
  </code></pre>
  
  <p>But I didn't read carefully so I performed it anyway. Now I can open Jupyter notebook, but my Python was downgraded to <code>2.7</code> How can I get my <code>Python 3.6</code> back?! :'( </p>
  
  <p>Thank you in advance!</p>