Tree drawing doesn't work on a headless server

[DavidNemeskey]

nltk.tree.Tree uses the Tcl (tkinter) backend to draw trees. Unfortunately, that doesn't seem to work on a headless server, which is the setup I use: run Jupyter on the server and access it via the browser from another machine.

If python3-tk is not installed, I get

ImportError: No module named '_tkinter', please install the python3-tk package

If it is, I get

TclError: no display name and no $DISPLAY environment variable

However, dot works in such a setup, so it would be preferable if it was used as the backend instead of tk. Also, it could be used to implement _repr_svg_() on Tree, which would also be a very welcome addition, as svg is a much better format for these kinds of things than png.

[rmalouf]

nltk.tree.Tree._repr_png_ also depends on ghostscript, which might be a problem. E.g., it doesn't get appear to get installed as part of the default anaconda environment.

[rawlins]

This is also an issue for running anything that might draw a tree through CI systems; e.g. I test my notebooks for a project on travis and was hitting it. The super hacky workaround I came up with for this scenario, where it's not really crucial that the tree get drawn, is at: rawlins/lambda-notebook@7f151a0

Edit: true to form, I figured out what I was missing within 30 seconds of posting here, so there's a slightly better (but still hacky) thing at the link above, vs. what I originally posted.

[anatoly-khomenko]

I managed to find a workaround for my configuration (Ubuntu 14 headless server), by installing python3-tk and xvfb and running jupyter notebook with xvfb-run.
Exact commands are:

sudo apt-get install python3-tk
sudo apt-get install xvfb
xvfb-run --auto-servernum --server-num=1 jupyter notebook --allow-root

I used the following code to test (note that tree.draw() will hang, as it is interactive):

from nltk.tree import Tree
from IPython.display import display

tree = Tree.fromstring('(S (NP this tree) (VP (V is) (AdjP pretty)))')
display(tree)

[rawlins]

I guess I never came back and posted in this issue, but I later ended up writing a pure python svg-based tree drawing package that works with nltk, and as a side-effect of being python-only also solves the problem of headless servers: https://github.com/rawlins/svgling

For Jupyter in particular, you will also need to fully disable nltk png-based trees, since Jupyter runs all available reprs in the background even though it shows only one; there's a snippet on that page showing how.

[pomkos]

The svgling package works great, but wondering if there is an inbuilt solution since 2019?

[rawlins]

This is more or less resolved by #2875 right? (If someone still wanted raster images in a headless setting, there's also now a straightforward conversion path to png via svgling+cairosvg.)

[DavidNemeskey]

I don't use nltk anymore, but for what it's worth, I agree. My original problem would have been solved by svgling.