Try this if your Python package installation fails

Problem

Ubuntu 18.04 comes with Python 3.6 preinstalled. Recently I checked out a private project that uses Pipfile and the project requires Python 3.5.

[requires]
python_version = "3.5"

I tried to install Django Compressor, but it failed because rjsmin and rcssmin could not be installed. I’ve found an existing issue which suggested installing the package with

pip install rjsmin --install-option="--without-c-extensions"

But Pipenv does not support this flag (at least not that I would know). And also it would make Compressor much slower. So instead I installed python3.5-dev:

sudo apt install python3.5-dev

and run

pipenv install django-compressor

again. It should install properly without any exceptions.

Conclusion

If any installation of Python packages fails, try installing Python header files for the correct version of Python that the virtual environment is using.