nbconvert fails with unicode preprocessor names

[juhasch]

I get the following error with nbconvert after installing custom preprocessors available in jupyter_contrib_nbextensions from conda-forge into a Python 2.7 environment:

Traceback (most recent call last):
  File "/home/juhasch/anaconda3/envs/py27/bin/jupyter-nbconvert", line 6, in <module>
    main()
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/jupyter_core/application.py", line 267, in launch_instance
    return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/traitlets/config/application.py", line 596, in launch_instance
    app.start()
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/nbconvert/nbconvertapp.py", line 293, in start
    self.convert_notebooks()
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/nbconvert/nbconvertapp.py", line 447, in convert_notebooks
    self.exporter = cls(config=self.config)
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/nbconvert/exporters/templateexporter.py", line 151, in __init__
    super(TemplateExporter, self).__init__(config=config, **kw)
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/nbconvert/exporters/exporter.py", line 101, in __init__
    self._init_preprocessors()
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/nbconvert/exporters/exporter.py", line 251, in _init_preprocessors
    self.register_preprocessor(preprocessor, enabled=True)
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/nbconvert/exporters/exporter.py", line 212, in register_preprocessor
    preprocessor_cls = import_item(preprocessor)
  File "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/ipython_genutils/importstring.py", line 31, in import_item
    module = __import__(package, fromlist=[obj])
TypeError: Item in ``from list'' not a string

It looks like this is a unicode problem with Python 2.7. This is the default jupyter_nbconvert_config.json file:

{
  "Exporter": {
    "template_path": [
      ".", 
      "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/jupyter_contrib_nbextensions/templates"
    ], 
    "preprocessors": [
      "jupyter_contrib_nbextensions.nbconvert_support.CodeFoldingPreprocessor",
      "jupyter_contrib_nbextensions.nbconvert_support.PyMarkdownPreprocessor"
    ]
  }, 
  "NbConvertApp": {
    "postprocessor_class": "jupyter_contrib_nbextensions.nbconvert_support.EmbedPostProcessor"
  }, 
  "version": 1
}

Adding r to the preprocessor strings makes it work again:

{
  "Exporter": {
    "template_path": [
      ".", 
      "/home/juhasch/anaconda3/envs/py27/lib/python2.7/site-packages/jupyter_contrib_nbextensions/templates"
    ], 
    "preprocessors": [
      r"jupyter_contrib_nbextensions.nbconvert_support.CodeFoldingPreprocessor",
      r"jupyter_contrib_nbextensions.nbconvert_support.PyMarkdownPreprocessor"
    ]
  }, 
  "NbConvertApp": {
    "postprocessor_class": "jupyter_contrib_nbextensions.nbconvert_support.EmbedPostProcessor"
  }, 
  "version": 1
}

Nbconvert version is 4.2.0 on Linux Mint.

[takluyver]

Won't adding an r prefix just make it invalid JSON, resulting in it not using that file at all?

[juhasch]

I don't think adding an 'r' is a solution. I get similar errors when I remove the prepocessor part, too.
So somehow the JSON files are seen as Unicode and ipython_genutils doesn't like that for Python 2.7.
Maybe it is a Linux Mint problem. This is where I get the error.

Any ideas what to look for ?

[takluyver]

JSON is conceptually unicode, and I think the JSON module leans towards returning parsed JSON strings as unicode. I guess you're probably just the first person to run into this: importable names listed in a JSON config file, parsed by Python 2.

[agreicius]

Thanks so much, @juhasch ! I had the exact same problems with nbconvert after installing the same nbextensions collection you mention, and after copying and pasting your edits all is good. (Probably not the correct way to use github, but it worked!)

[mpacer]

Am I correct in saying that #370 addresses this?

[agreicius]

Not sure if @michaelpacer 's question was directed at me, but to be specific, after making the changes described in the following two commits, the problem was fixed/

1. ipython/ipython_genutils@3d47e88
2. f01e44d

[arogozhnikov]

Not sure if @michaelpacer 's question was directed at me, but to be specific, after making the changes described in the following two commits, the problem was fixed/

Thanks a lot, this fix was enough for me:

1. ipython/ipython_genutils@3d47e88