If I have two versions of python3 installed on my machine (ie: 3.11.2 and 3.12), how do I set the default version to use when running python scripts?

Question

I installed the earlier version (3.11.2) normally using apt package manager and the later version (3.12) manually from source. When I execute python scripts like so:

./python_script.py

The script uses the earlier version instead of the later one. I would like to leave the default shebang line #!/usr/bin/env python3 at the beginning of the script alone for portability reasons.

Here is a sample test script.

#!/usr/bin/env python3

import sys

def main():
  print(sys.version)

main()

EDIT: With the above configuration, I'm thinking at this point the quickest fix (probably not the best because of future consequences, please see well explained answers and comments from the community below) is to change the target of the python3 symlink in /etc/alternatives to /usr/local/bin/python3.12 where my python3.12 binary is stored.

EDIT2: As seen in the second screenshot, I was able to get ./myscript.py to use the python version I wanted (3.12) by verifying that the /usr/local/bin directory is before the /usr/bin directory in my PATH environment variable and renaming the python3.12 binary in /usr/local/bin/ to python3.

Answer 1

Since you’re using env python3, the default version of Python will be determined by whichever version of python3 is first in the PATH variable. You can thus choose which version is used by constructing your PATH variable appropriately: if you want the system default, ensure /usr/bin appears before any other directory containing python3, and if you want your own build of Python, ensure the directory containing that version of python3 appears before /usr/bin.

Your proposed solution is one of the worst you could apply at this point. Given the situation shown in your screenshot, you haven’t broken your system yet: system Python scripts, which specify #!/usr/bin/python3 in their shebang, can still find the modules they expect. If you change /usr/bin/python3 to point to Python 3.12, those scripts will lose access to the 3.11 modules installed on your system; in particular, python3-apt. This is liable to have unfortunate consequences.

Since you are specifying #!/usr/bin/env python3 in your own scripts, and can therefore rely on PATH, I still think a PATH-based approach is better in your circumstances. You can even change PATH everywhere, to point to your python3 first, without breaking system Python scripts (since they don’t rely on PATH to find Python).

Answer 2

To complement @StephenKitt's fine answer, on Debian-based systems, you may be able to install packages for more than one version of python concurrently. For instance here on Ubuntu 22.04, I have 3.10 and 3.11 installed:

$ aptitude search -F%p '~i ~n "^python[0-9.]*$"'
python3
python3.10
python3.11

The python3 package depends on python3.10 and the python3 executable is a symlink to python3.10.

$ zstat +link --  ${(v)commands[(I)python[0-9.]#]}
/usr/bin/python3 python3.10
/usr/bin/python3.10
/usr/bin/python3.11

Do not change that symlink manually. python versions are often incompatible between each other, and python modules are not shared between python versions, so changing that python3 symlink will break the scripts that have a #! /usr/bin/env python3 or #! /usr/bin/python3 (or #! /bin/python3 as /bin is often a symlink to /usr/bin nowadays) and expect python3 to be 3.10 and have access to the python modules installed for 3.10.

So if you need to invoke a script that needs a different installed version of python, and you can't change the shebang to #! /usr/bin/python3.12, either just invoke it as:

python3.12 /path/to/that-file.py

Or create a:

mkdir ~/.python3-is-python3.12
ln -s /usr/bin/python3.12 ~/.python3-is-python3.12/python3

And put ~/.python3-is-python3.12 ahead in your $PATH but only for the invocation of that script, setting it globally would likely break other scripts that use #! /usr/bin/env python3 and expect python3 to be the system's default python3.

Answer 3

I know this questions only asks about the apt package manager, and the other answers cover that aspect very well.

But if you are working with several different python projects, all requiring different python versions and/or different dependencies, I would strongly recommend using a virtual environment (venv) to manage the python versions and the dependencies of your projects.

To install a virtual environment for a project, e.g. for 3.11, In the project's main directory, run:

python3.11 -m venv venv

This will create a directory named venv containing the virtual environment.

Then, to use the environment, type:

. venv/bin/activate

From that point on, in the current shell you will be able to run python and pip without the version number, and the correct environment will be used. To get out of the environment, type deactivate.

In the new environment you will have only the pip dependencies that you installed inside it, so you won't have any dependency mismatches between projects. You will also be able to easily create the requirements.txt file for the project with pip freeze > requirements.txt after you have installed the dependencies.

Answer 4

Since you're using Debian (or one of its derivatives), you could consider using the update-alternatives utility.

For example to set the default to your 3.12 version residing in (for example) /usr/local/bin/:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/local/bin/python3.12 10

The response should be something like this (I've not actually installed Python 3.12):

update-alternatives: using /usr/local/bin/python3.12 to provide /usr/bin/python3 (python3) in auto mode

See man update-alternatives for details, and other options.