Optional dependencies in a pip requirements file

Instead of specifying optional dependencies in the same file as the hard requirements, you can create a optional-requirements.txt and a requirements.txt.

To export your current environment's packages into a text file, you can do this:

pip freeze > requirements.txt

If necessary, modify the contents of the requirements.txt to accurately represent your project's dependencies. Then, to install all the packages in this file, run:

pip install -U -r requirements.txt

-U tells pip to upgrade packages to the latest version, and -r tells it to install all packages in requirements.txt.

In 2015 PEP-0508 defined a way to specify optional dependencies in requirements.txt:

requests[security]

That means that yourpackage needs requests for its security option. You can install it as:

pip install yourpackage[security]

You are misunderstanding the documentation; it's not as clear as it could be. The point in the documentation is that with a requirements file you can feel free to specify your full recommended working set of packages, including both necessary dependencies and optional ones.

You can add comments (lines beginning with #) to distinguish the two to humans, but pip makes no distinction. You can also have two requirements files, as Daniel suggests.