Debian_Ubuntu_Alternative_Versions_System_101.md

Based on https://chatgpt.com/share/67b9ff44-bd6c-8007-84f4-a51d8dddfd95

You can switch between the registered Python versions using the update-alternatives interactive selection tool. Here’s how to do it:

Run the Configuration Command:

sudo update-alternatives --config python3

Select the Desired Version:

After running the command, you’ll see a list similar to this:

There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /usr/bin/python3.9   2         auto mode
  1            /usr/bin/python3.8   1         manual mode
  2            /usr/bin/python3.9   2         manual mode

Press <enter> to keep the current choice[*], or type selection number:

Enter the Number:

Type the number corresponding to Python 3.8 (in this example, it might be 1) and press Enter. This action will change the default python3 to Python 3.8.
Verify the Change:

Check the currently active Python version with:
```
python3 --version
```

This process tells the alternatives system to update the symbolic link for /usr/bin/python3 to point to your chosen version.

E.g. How to Setup python3.8 and python3.9 and switch betwen them (Debian, Ubuntu)

Steup python3.8 and python3.9 as alternatives

# sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 1
# sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 2

(last digits are priorities - to be adjusted based on your needs, higher value = higher priority , so in above case "pythion 3.9" has priority 2 and has higher prioryt then python 3.8)

Examplle of `sudo add-apt-repository universe` not working with pyehon3.9

By default it's python 3.9 in this setup but let's switch to showcase how to do switchgin:

switch to "(auto)" pointing to python3.9

# sudo update-alternatives --config python3
There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /usr/bin/python3.9   2         auto mode
* 1            /usr/bin/python3.8   1         manual mode
  2            /usr/bin/python3.9   2         manual mode

Press <enter> to keep the current choice[*], or type selection number: 0
update-alternatives: using /usr/bin/python3.9 to provide /usr/bin/python3 (python3) in auto mode

Example command not workign with python3.9 (at least as of 2025-02-22):

# sudo add-apt-repository universe
Traceback (most recent call last):
  File "/usr/bin/add-apt-repository", line 12, in <module>
    from softwareproperties.SoftwareProperties import SoftwareProperties, shortcut_handler
  File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 68, in <module>
    from gi.repository import Gio
  File "/usr/lib/python3/dist-packages/gi/__init__.py", line 42, in <module>
    from . import _gi
ImportError: cannot import name '_gi' from partially initialized module 'gi' (most likely due to a circular import) (/usr/lib/python3/dist-packages/gi/__init__.py)

Swtiching to python3.8 and working command

Now let's switch to python3.8 :

# sudo update-alternatives --config python3
There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.9   2         auto mode
  1            /usr/bin/python3.8   1         manual mode
  2            /usr/bin/python3.9   2         manual mode

Press <enter> to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/bin/python3.8 to provide /usr/bin/python3 (python3) in manual mode

and command works:

# sudo add-apt-repository universe
'universe' distribution component is already enabled for all sources.

Cleanup: Now let's switch back to "(auto)" mode;

# sudo update-alternatives --config python3
There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /usr/bin/python3.9   2         auto mode
* 1            /usr/bin/python3.8   1         manual mode
  2            /usr/bin/python3.9   2         manual mode

Press <enter> to keep the current choice[*], or type selection number: 0
update-alternatives: using /usr/bin/python3.9 to provide /usr/bin/python3 (python3) in auto mode

gwpl/Debian_Ubuntu_Alternative_Versions_System_101.md