When would be -e, --editable option useful with pi

Question

When would the -e, --editable option be useful with pip install?

For some projects last line in requirements.txt is -e .. What does it do exactly?

Answer 1

As the man page says it:

-e,--editable <path/url>
     Install a project in editable mode (i.e.  setuptools "develop mode") from a local project path or a VCS url.

So you would use this when trying to install a package locally, most often in the case when you are developing it on your system. It will just link the package to the original location, basically meaning any changes to the original package would reflect directly in your environment.

Some nuggets around the same here and here.