Changing the default python without changing the $PATH variable?

Question

I want to change the default python version I use, without affecting any other defaults. Appending the python path to the $PATH environment variable only accomplishes that as long as there is only the python executable at that given location, which is not the case if I want to use the /usr/bin version (export PATH=\usr\bin:$PATH).

I know I could theoretically create a symbolic link in some other folder of my choosing, but this is not very elegant.

Is there any other way of changing the default python (like a nice environment variable that python uses which takes precedence over the $PATH environment variable)?

• GadaaDhaariGeek: You can't change default python version in Linux (I assume you are using linux machine) as there are dependencies involved. What you can do is that, you can create an alias of python command.
• scapiskipi: I am not sure if I expressed myself correctly, but what I meant was to change the result of the command which python and consequently which python version is used when I type python in the terminal, which is already accomplished by appending the path to the desired python version to the PATH environment variable.

Answer 1

If what you want is to get a specific version of the Python interpreter when you type python in your shell, there is no environment variable that can help you in that sense. It is the shell that decides the binary of the interpreter to use, and by the time it is spawned it cannot be swapped to a different one. Some ways that you can work around this:

• As you said, making a symlink somewhere. It does not need to be in a system directory, many users have something like $HOME/bin in their $PATH (some Linux distributions do this by default).
• Make an alias (e.g. in your .bashrc or similar).
• Make a function (same as previous). You could even program it to receive some argument that decides which version of the interpreter you want to use, or to use the value of some environment variable, as you suggested.
• In distributions supporting it, set up an alternative.
• Make a virtual environment using the interpreter that you want and work within that environment.

In any case, there are a few things you should take into account:

• As mentioned in another answer, changing the version of the Python interpreter system-wide may break things. Particularly, switching the command python from Python 2 to Python 3 (or the other way around) is almost guaranteed to cause quite a lot of trouble. Some of the solutions above may be unaffected by this (e.g. setting and alias or function in .bashrc should not affect other scripts, unless sourced), but you should be careful about it.
• A common pitfall here is that changing the python command does not change every Python-based command. For example, if you use IPython and just type ipython you need to make sure that the correct interpreter and script are launched. Depending on your context there may be a few tools that you may need to consider in that sense.

A virtual environment is probably the easiest and cleanest solution, since it was designed for that particular problem (even though issues still arise sometimes, e.g. if you run ipython but you forgot to install it in your environment first you will get the system-wide one, obviously), although if you want to use it for every shell session then it may not be as convenient.

Answer 2

Depending on what OS you're using, you can use alternatives to change the default.

https://wiki.debian.org/DebianAlternatives https://linuxconfig.org/how-to-switch-between-python-versions-on-fedora-linux

So, to change the default Python, you could do something like

alternatives --install /usr/bin/python python /usr/bin/python3.6 1

Be aware though, this can break a lot of stuff as the OS is likely dependent on the version of Python.

• scapiskipi: Sounds neat! And I use Debian, so it is installed. But I do not have admin rights, and it's a shared system, so I cannot use this solution.

Answer 3

In general, Python itself does not provide any way to do this. Once Python starts up, there is already one particular version running, and it can't change to a different version on the fly.

Your operating system may provide some way of choosing which version of Python is the default (e.g. eselect python on Gentoo Linux), but it's impossible to say whether that's the case for you without knowing what OS you're using. If your OS doesn't provide something like this, it is possible to make your own scripts to set and change a default Python version.

• scapiskipi: Oh, I see. I am using Debian, but I do not have any admin rights, so I doubt that an OS solution could help.