Thanks @Wilfred.

Regarding using a set, get_installed_distributions iterates over the pkg_resources working set, which is a reprensentation of the currently installed packages, a list behind the scenes (https://bitbucket.org/pypa/setuptools/src/eb58c3fe7c0881cbb28de1c523f083ad8e7f427d/pkg_resources.py?at=default#cl-542). Being able to retain all information about this working set is very useful when you want to debug a situation with setuptools since it may add packages to that working set in a specific order due to the use of egg files et al. In other words, this subcommand should retain the order of requirements to be closer to the underlying technology.