From python pip install -h:
-e, --editable <path/url> Install a project in editable mode (i.e. setuptools
"develop mode") from a local project path or a VCS url.
Basically , if you pip install -e <package_dir>, pip will install a python package, and symlink to the package, instead of copying it's contents. This is very useful, because it means that you can edit the package, and the changes are available immediately, without having to re-install the package after every modification.
Is there an equivalent for R packages? This would be extremely useful for package development.