The issue is that the units examples (https://github.com/matplotlib/matplotlib/tree/master/examples/units) import from a local module that provides the unit classes and handlers. However, it turns out that this was working coincidentally due to a sorta-nasty bug where if you start python via python -m some_module (which is how sphinx gets launched internally) '' is put in the path, which ends up not getting expanded so the cwd is always in the path (see https://bugs.python.org/issue33053 and https://docs.python.org/3/whatsnew/3.7.html#other-language-changes).

This means the docs do not build cleanly any more 😞 .

If you run the examples from the cli they work correctly. If you explicitly add the cwd back into the path they also build fine.

Not sure what the best solution here is.