title: How can I fix "500 : Internal Server Error" on jupyter notebook due to 'tornado.web' not having attribute 'asynchronous'?
body: <p>Jupiter notebook is not opening notebooks and shows a "500 : Internal Server Error" instead. What I did was activate an environment I need and launching Jupyter. Please find below what I did and the error message on the terminal:</p>
  
  <pre><code>$ conda activate py3_parcels
  
  $ jupyter notebook [I 18:20:39.663 NotebookApp] Loading IPython parallel extension [I 18:20:39.665 NotebookApp] Serving notebooks from local directory: /Users/davidlindo-atichati/run/GOM_MED [I 18:20:39.665 NotebookApp] The Jupyter Notebook is running at: [I 18:20:39.665 NotebookApp] http://localhost:8888/?token=36466fc830862b794856668cd6ded86fe0a664247233764a [I 18:20:39.665 NotebookApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation). [C 18:20:39.677 NotebookApp] 
  
      Copy/paste this URL into your browser when you connect for the first time,
      to login with a token:
          http://localhost:8888/?token=36466fc830862b794856668cd6ded86fe0a664247233764a [I 18:20:40.211 NotebookApp] Accepting one-time-token-authenticated connection from ::1
  </code></pre>
  
  <p>The notebook opens on my Safari browser. But when I try to open an existing notebook or when I create a new notebook I receive a "500 : Internal Server Error" in the browser and the following message on the terminal:</p>
  
  <pre><code>[E 18:22:16.169 NotebookApp] Uncaught exception GET /notebooks/backtracking-sample-1.ipynb (::1)
      HTTPServerRequest(protocol='http', host='localhost:8888', method='GET', uri='/notebooks/backtracking-sample-1.ipynb', version='HTTP/1.1', remote_ip='::1')
      Traceback (most recent call last):
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/tornado/web.py", line 1697, in _execute
          result = method(*self.path_args, **self.path_kwargs)
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/tornado/web.py", line 3174, in wrapper
          return method(self, *args, **kwargs)
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/notebook/notebook/handlers.py", line 59, in get
          get_custom_frontend_exporters=get_custom_frontend_exporters
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/notebook/base/handlers.py", line 467, in render_template
          return template.render(**ns)
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/jinja2/asyncsupport.py", line 76, in render
          return original_render(self, *args, **kwargs)
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/jinja2/environment.py", line 1008, in render
          return self.environment.handle_exception(exc_info, True)
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/jinja2/environment.py", line 780, in handle_exception
          reraise(exc_type, exc_value, tb)
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/jinja2/_compat.py", line 37, in reraise
          raise value.with_traceback(tb)
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/notebook/templates/notebook.html", line 1, in top-level template code
          {% extends "page.html" %}
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/notebook/templates/page.html", line 154, in top-level template code
          {% block header %}
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/notebook/templates/notebook.html", line 120, in block "header"
          {% for exporter in get_custom_frontend_exporters() %}
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/notebook/notebook/handlers.py", line 19, in get_custom_frontend_exporters
          from nbconvert.exporters.base import get_export_names, get_exporter
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/nbconvert/__init__.py", line 7, in &lt;module&gt;
          from . import postprocessors
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/nbconvert/postprocessors/__init__.py", line 5, in &lt;module&gt;
          from .serve import ServePostProcessor
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/nbconvert/postprocessors/serve.py", line 19, in &lt;module&gt;
          class ProxyHandler(web.RequestHandler):
        File "/Users/davidlindo-atichati/miniconda3/envs/py3_parcels/lib/python3.7/site-packages/nbconvert/postprocessors/serve.py", line 21, in ProxyHandler
          @web.asynchronous
      AttributeError: module 'tornado.web' has no attribute 'asynchronous' [E 18:22:16.178 NotebookApp] {
        "Host": "localhost:8888",
        "Cookie": "username-localhost-8888=\"2|1:0|10:1551828040|23:username-localhost-8888|44:OGQ4ZTgzZDRjZjFiNDMzZDk0ZTI2NmJkMDc3OTEzMDk=|e4504cac3111242d53ab46ce3b8fa72750876a90bef7bc980a2ebb3c6f77caec\"; username-localhost-8889=\"2|1:0|10:1551821266|23:username-localhost-8889|44:ZjgyOTBhNTE3Zjg0NDJlOGIyNTA5NDRiNTBjMzQ0YTk=|43b122243108c4f7dbf37e636660b8d0656f5173d2b363a6bb496681de6d4e5f\";
  _xsrf=2|0a22b984|8687c792155e13f5d476952db2d9f2f7|1551360338",
        "Connection": "keep-alive",
        "Upgrade-Insecure-Requests": "1",
        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
        "User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_3) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0.3 Safari/605.1.15",
        "Referer": "http://localhost:8888/tree",
        "Accept-Language": "en-us",
        "Accept-Encoding": "gzip, deflate"
      } [E 18:22:16.178 NotebookApp] 500 GET /notebooks/backtracking-sample-1.ipynb (::1) 121.50ms referer=http://localhost:8888/tree
  </code></pre>