I encountered the issue today and I believe ignore installed should not be enforced. If the system already has a package that match a condition, it should be used instead of installing a new one from pypi. I think that is the behavior that most would expect.
If there is a version requirement that is not matched by the system package, it will end up being installed in the user directory. A way to cancel the patch is to configure pip via an environment variable: PIP_IGNORE_INSTALLED=false An example is GitPython. The available version as of this writing: Stretch: 2.1.1 testing: 2.1.8 Pypi: 2.1.9 Given python3-git is installed and running pip3 install GitPython, the version 2.1.9 from pypi is installed. As a root user, it find the existing system installation. At a minimum README.Debian could better highlight that behavior and the pip install helps could add more details to --ignore-installed: -I, --ignore-installed Ignore the installed packages (reinstalling instead). On Debian: always set for non root users (system packages are ignored). Set PIP_IGNORE_INSTALLED=false to override. But probably it would be nicer to no more hardcode --ignore-installed and let the user pick the best decision. -- Antoine Musso
