A.2 The “system” Python

On many operating systems, Python may already be installed because the system needs it for some of its own tasks. This Python installation is often referred to as the “system” Python. On macOS, for example, Python 2.7 is installed and available for you to use.

Using the system Python gets tricky when you need to install packages, because they will all get installed under this global version of Python. If you install a package that overrides something the operating system needs, or you have multiple projects that need different versions of a package, bad things can start to happen. I strongly recommend avoiding the system Python.