Embedding Bokeh server in Flask

I posted the whole text with error messages included on stackoverflow.com (because the code there it is a lot more readable than here; on Google Groups I also have problems setting tags…)
https://stackoverflow.com/questions/45414715/embedding-bokeh-server-as-a-library-tornado-error

I am following Bokeh’s User Guide.

In “Embedding Bokeh Server as a Library” at http://bokeh.pydata.org/en/latest/docs/user_guide/server.html#embedding-bokeh-server-as-a-library
it refers to a demo where a Bokeh server is embedded in Flask (https://github.com/bokeh/bokeh/blob/0.12.6/examples/howto/server_embed/flask_embed.py)

It should be straightforward but I get a Tornado error if launched with python flask_embed.py.

Anybody has an idea WHY?

This is the whole traceback:

Opening Flask app with embedded Bokeh application on http://localhost:8080/
ERROR:tornado.application:Uncaught exception GET /bkapp/autoload.js?bokeh-autoload-element=3a711948-3668-4f63-8d0c-8cd1584fb92d&bokeh-app-path=/bkapp&bokeh-absolute-url=http://localhost:5006/bkapp (127.0.0.1)
HTTPServerRequest(protocol='http', host='localhost:5006', method='GET', uri='/bkapp/autoload.js?bokeh-autoload-element=3a711948-3668-4f63-8d0c-8cd1584fb92d&bokeh-app-path=/bkapp&bokeh-absolute-url=http://localhost:5006/bkapp', version='HTTP/1.1', remote_ip='127.0.0.1', headers={'Accept-Language': 'en-US,en;q=0.5', 'Accept-Encoding': 'gzip, deflate', 'Host': 'localhost:5006', 'Accept': '*/*', 'User-Agent': 'Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:53.0) Gecko/20100101 Firefox/53.0', 'Connection': 'keep-alive', 'Referer': 'http://localhost:8080/', 'Cookie': 'username-localhost-8888="2|1:0|10:1501067928|23:username-localhost-8888|44:Y2EwOTUzN2YzNWRiNGQyMDgxZWEyOGMzZDJkOTI4ZWY=|f4f981dd915dc777c70e605b7135bcbbc076b3fe3482999e5ca557cb4abd518e"; _xsrf=2|c711b8e7|f913ccc5c9cc32532c1e67bbd75b6051|1500889250'})
Traceback (most recent call last):
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/web.py", line 1511, in _execute
result = yield result
File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1055, in run
value = future.result()
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/concurrent.py", line 238, in result
raise_exc_info(self._exc_info)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1063, in run
yielded = self.gen.throw(*exc_info)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/bokeh/server/views/autoload_js_handler.py", line 31, in get
session = yield self.get_session()
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1055, in run
value = future.result()
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/concurrent.py", line 238, in result
raise_exc_info(self._exc_info)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1063, in run
yielded = self.gen.throw(*exc_info)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/bokeh/server/views/session_handler.py", line 40, in get_session
session = yield self.application_context.create_session_if_needed(session_id, self.request)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1055, in run
value = future.result()
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/concurrent.py", line 238, in result
raise_exc_info(self._exc_info)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1069, in run
yielded = self.gen.send(value)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/bokeh/server/application_context.py", line 177, in create_session_if_needed
self._application.initialize_document(doc)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/bokeh/application/application.py", line 121, in initialize_document
h.modify_document(doc)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/bokeh/application/handlers/function.py", line 16, in modify_document
self._func(doc)
  File "main.py", line 22, in modify_doc
df = pd.read_csv(data_url, parse_dates=True, index_col=0)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/pandas/io/parsers.py", line 655, in parser_f
return _read(filepath_or_buffer, kwds)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/pandas/io/parsers.py", line 392, in _read
filepath_or_buffer, encoding, compression)
  File "/home/alessandro/git-files/python/study_graph2/env/local/lib/python2.7/site-packages/pandas/io/common.py", line 186, in get_filepath_or_buffer
req = _urlopen(url)
  File "/usr/lib/python2.7/urllib2.py", line 154, in urlopen
return opener.open(url, data, timeout)
  File "/usr/lib/python2.7/urllib2.py", line 435, in open
response = meth(req, response)
  File "/usr/lib/python2.7/urllib2.py", line 548, in http_response
'http', request, response, code, msg, hdrs)
  File "/usr/lib/python2.7/urllib2.py", line 473, in error
return self._call_chain(*args)
  File "/usr/lib/python2.7/urllib2.py", line 407, in _call_chain
result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 556, in http_error_default
raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
HTTPError: HTTP Error 400: Bad Request
ERROR:tornado.access:500 GET /bkapp/autoload.js?bokeh-autoload-element=3a711948-3668-4f63-8d0c-8cd1584fb92d&bokeh-app-path=/bkapp&bokeh-absolute-url=http://localhost:5006/bkapp (127.0.0.1) 425.75ms