Update install instructions with note about using conda and pip together about message_ix HOT 2 CLOSED

AttributeError: 'message_ix' has no attribute 'Scenario'

I know I've seen these before and I don't know if the cause is always the same.

AFAICT, this happens in the following case:

• User first makes a regular install of message_ix (whether from a local directory or from PyPI or GitHub directly).
• User next wants to switch to using an editable install from local source, e.g. a Git repo directory.
• User does pip install --editable ..

What happens in this case is that the editable install files are created in their site-packages/ directly, but an empty directory site-packages/message_ix/ is left behind from the regular install. IOW, pip removes the files but not the directory for the existing, regular install.

When the user next tries to import message_ix, Python finds the empty directory site-pacakges/message_ix/ and imports it, but this contains absolutely nothing, hence the API is not usable.

The fix I've tried is to first pip uninstall message-ix before pip install --editable .. This has worked in all cases I've encountered (regardless of whether users are using conda, venv, etc. or not).

from message_ix.

Great, thanks! We can include this possible fix for the issue in a Known issue with the same PR :)

from message_ix.