During development of our Python/Django application, we accidentally introduced package requirements in requirements.txt that installed conflicting versions of a library. Heroku has cached the broken requirements in the CACHE_DIR specified by the python buildpack, as documented on their buildpack API info page, and it doesn't seem that any amount of modification to the requirements.txt file can purge the cache of the broken packages. How can we force Heroku to totally empty out the CACHE_DIR and reinstall all requirements and dependencies from scratch?
I ran in the same problem as I was trying to install a module (django-piston) from source as the last official release is a bit old and lacks some bug fixes that are crucial to me. Anyway, there has been an issue opened before, and I actually got a reply from support.
It seems a bit odd to me, but at least it works (just choose any python release number in runtime.txt, rebuild, and then change back to your desired runtime and rebuild again)