Newest version of Tornado has compatibility issue

[WillTheSun]

I just did a fresh install and ran into an issue with Tornado. I was able to fix by downgrading Tornado to version 4.1.

$ python cps.py
Traceback (most recent call last):
  File "cps.py", line 18, in <module>
    from tornado.wsgi import WSGIContainer
  File "/Users/williamsun/Projects/calibre-web/vendor/tornado/wsgi.py", line 41, in <module>
    from tornado import web
  File "/Users/williamsun/Projects/calibre-web/vendor/tornado/web.py", line 86, in <module>
    from tornado import gen
  File "/Users/williamsun/Projects/calibre-web/vendor/tornado/gen.py", line 112, in <module>
    from singledispatch import singledispatch  # backport
ImportError: No module named singledispatch

[redcatone]

I encountered the same error but was able to fix it by installing the required module.
python -m pip install singledispatch backports_abc
Answer from Stack Overflow.

[GAumala]

I was having this same issue when using pip for Python 3. When I switched to pip for Python 2 everything worked flawlessly.

[WillTheSun]

Interesting, I was also using pip for Python3. Not sure why it breaks