Wandering Thoughts archives

It's unfortunately time to move away from using '/usr/bin/python'

For a long time, the way to make Python programs runnable on Unix has been to start them with '#!/usr/bin/python' or sometimes '#!/usr/bin/env python' (and then chmod them executable, of course; this makes them scripts). Unfortunately this is no longer a good idea for general Python programs, for the simple reason that current Unixes now disagree on what version of Python is '/usr/bin/python'. Instead, we all need to start explicitly specifying what version of Python we want by using '/usr/bin/python3' or '/usr/bin/python2' (or by having env explicitly run python3 or python2).

For a long time, even after Python 3 came out, it seemed like /usr/bin/python would stay being Python 2 in many environments (ones where you had Python 2 and Python 3 installed side by side). I expected a deprecation of /usr/bin/python as Python 2 to take years after Python 2 itself was no longer supported, for the simple reason that there are a lot of programs and instructions out there that expect their '#!/usr/bin/python' or 'python' to run Python 2. Changing what that meant seemed reasonably disruptive, even if it was the theoretically correct and pure way.

In reality, as I recently found out, Fedora 31 switched what /usr/bin/python means, and apparently Arch Linux did it several years ago. In theory PEP 394 describes the behavior here and this behavior is PEP-acceptable. In practice, before early July of 2019, PEP 394 said that 'python' should be Python 2 unless the user had explicitly changed it or a virtual environment was active. Then, well, there was a revision that basically threw up its hands and said that people could do whatever they wanted to with /usr/bin/python (via).

(This makes PEP 394 a documentation standard. As with all documentation standards, it needs to describe reality to be useful, and the reality is that /usr/bin/python is now completely unpredictable.)

Since Fedora and Arch Linux have led the way here, other Linux distributions will probably follow. In particular, since Red Hat Enterprise is more or less based on Fedora, I wouldn't be surprised to see RHEL 9 have /usr/bin/python be Python 3. I don't think Debian and thus Ubuntu will be quite this aggressive just yet, but I wouldn't be surprised if in a couple of years /usr/bin/python at least defaults to Python 3 on Ubuntu. (Hopefully Python 2 will still be available as a package.)