[westurner]

Pip supports constraints files. https://pip.pypa.io/en/stable/user_guide/#constraints-files :

> Constraints files are requirements files that only control which version of a requirement is installed, not whether it is installed or not. Their syntax and contents is nearly identical to Requirements Files. There is one key difference: Including a package in a constraints file does not trigger installation of the package.

> Use a constraints file like so:

  python -m pip install -c constraints.txt

[jshen]

What does that command do? Does it install dependencies, or simply verify the versions of the installed dependencies?

[ptx]

It just gives you an error:

  ERROR: You must give at least one requirement to install (see "pip help install")

So it seems like a strange choice of usage example. You have to provide both requirements and constraints for it to do anything useful (applying the version constraints to the requirements and their dependencies).