Module not found Heroku

There are some bugs and problems that give a thrill once they're solved. The best bugs are the ones that teach you something, the worst bugs are the ones that indicate that you've not improved your spelling and the difference between an l and a 1 is large.

Figuring out why I was facing the following traceback in heroku provided a time consuming learning experience, but probably one that I won't forget.

heroku[web.1]: State changed from crashed to starting
heroku[web.1]: State changed from starting to crashed
heroku[web.1]: State changed from crashed to starting
app[web.1]: Traceback (most recent call last):
app[web.1]: File "/home/app/server/bin/server", line 3, in <module>
app[web.1]: from api.server import deploy
app[web.1]: ModuleNotFoundError: No module named 'api'

Why can't the module be found 🤔!

Worst of all, the dockerfile builds locally, and when I run it. It's executing /home/app/server/bin/server and ready to receive traffic...

FROM python:3.8.2

ENV USER appuser
ENV HOME /home/${USER}

RUN mkdir -p ${HOME}/server
WORKDIR ${HOME}/server

ENV PATH ${HOME}/server/bin:${HOME}/.local/bin:$PATH

USER appuser

CMD ["server"]

I won't go into how long I spent on thinking it had something to do with permission. Looking back, it's quite clear that a permission has nothing to do with it, since the logs would say so.

Take a step back

The difference between the image on heroku and the image running locally. What probably made me assume it was a permission error was the quote from their docs:

containers are not run with root privileges in Heroku

So there was some funky business they're doing to the user I provided.

Get closer to the problem

Finding out that I could get inside the heroku container certainly helped me figure out the problem:

heroku run bash

I could now recreate the Module not found error. I tried using pipenv to install the module that was missing, however that didn't work either. hmm.. Where are these modules installed??

Show me the site-packages:

python -m site


sys.path = [
USER_BASE: '/home/appuser/server/.local' (exists)
USER_SITE: '/home/appuser/server/.local/lib/python3.8/site-packages' (exists)

Local Docker Container

sys.path = [
USER_BASE: '/home/appuser/.local' (exists)
USER_SITE: '/home/appuser/.local/lib/python3.8/site-packages' (exists)

Right so they are not referencing the same site-packages. Are the site packages even installed in the heroku container?!?

$ ls /home/appuser/.local/lib/python3.8/site-packages
Flask-1.1.2.dist-info               mccabe-0.6.1.dist-info
Flask_Cors-3.0.8.dist-info          more_itertools
Flask_JWT_Extended-3.24.1.dist-info more_itertools-8.2.0.dist-info
Flask_SQLAlchemy-2.4.1.dist-info    oauthlib

🎉 so they aren't missing...


echo $HOME


$ echo $HOME

Ah.. So $HOME has something to do with it.

Well I have $HOME=/home/appuser and WORKDIR=/home/appuser/server perhaps heroku is setting the work directory to the home directory. 🤷‍♂️

ENV USER hints
ENV HOME /home/${USER}

RUN mkdir -p ${HOME}

And sure enough fixing my deployment.