"Don't install on the system Python"

[Chris Angelico]

On Sun, Dec 1, 2019 at 3:46 PM John Ladasky <john_ladasky at sbcglobal.net> wrote:
>
> Long-time Ubuntu user here.
>
> For years, I've read warnings about not installing one's personal stack of Python modules on top of the system Python.  It is possible to corrupt the OS, or so I've gathered.
>
> Can anyone provide concrete examples of problems arising from installing modules on top of the system Python?  Am I courting disaster?
>

I'm going to start by separating out two concepts that are often, but
not always, the same.

The "system Python" is the one that the OS depends on. On my Debian
Stretch, that's /usr/bin/python3 (symlink to /usr/bin/python3.5). On
older systems, that might be a Python 2.7 (or worse). This is the
installation of Python that is managed by your OS package manager, and
- more importantly - is the one that any OS-provided scripts will
depend on.

The "default Python" is the one you get when you type "python3" at the
shell. Often this is the same as the system Python, but there's no
requirement for this to be the case. On many of my systems, I compile
and install a new build of Python periodically (usually from the
master branch, so it's a pre-alpha), and I'm happy for that to take
over the name "python3". Currently, on my main system (the
aforementioned Debian Stretch), that's /usr/local/bin/python3 (a
symlink to /usr/local/bin/python3.9).

Since the system Python is managed by your OS package manager, you
need to be careful about using pip to install packages into it. For
instance, if I were to "/usr/bin/python3 -m pip install psycopg2", it
would potentially conflict with "sudo apt install python3-psycopg2".
You MAY be safe using pip to install something that isn't available in
your package manager, but it's possible to run into dependency
versioning conflicts. Be careful.

But installing into your default Python, if it's not the system
Python, is absolutely safe. Or rather, if it breaks anything, then
it's not your fault - it's the fault of something depending on the
system Python but not using an absolute shebang :)

ChrisA