Ubuntu how to change from python3.10 to python3.9?

Question

I have python 3.10 installed. But I am trying to use a python script which is only compatible with python 3.9.

So I have installed 3.9.

wget https://www.python.org/ftp/python/3.9.0/Python-3.9.0.tar.xz
tar -xf Python-3.9.0.tar.xz

cd Python-3.9.0

./configure

sudo make altinstall

Python 3.9 is now installed, so I have bother 3.9 and 3.10 installed. When I search how to use 3.9 most results say to delete 3.10. Is there an option to temprarily switch to 3.9? So after can switch back to 3.10?

Thanks

Answer 1

When you have:

$ ls -l /usr/bin/python3.10
-rwxr-xr-x 1 root root 5921160 Nov 14 17:10 /usr/bin/python3.10
$ ls -l /usr/local/bin/python3.9
-rwxr-xr-x 1 root root 15651728 Dec 31 15:58 /usr/local/bin/python3.9

Then you can do:

sudo update-alternatives --install /usr/bin/python3 python /usr/local/bin/python3.10 0
sudo update-alternatives --install /usr/bin/python3 python /usr/local/bin/python3.9 0

After this being done, this should work:

$ python3
Python 3.10.6 (main, Nov 14 2022, 16:10:14) [GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> quit()
$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python3).

  Selection    Path                      Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.10        0         auto mode
  1            /usr/bin/python3.10        0         manual mode
  2            /usr/local/bin/python3.9   0         manual mode

Press <enter> to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/local/bin/python3.9 to provide /usr/bin/python3 (python) in manual mode
$ python3
Python 3.9.0 (default, Dec 31 2022, 15:24:44)
[GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> quit()
$

After the update-alternatives process, you should find this link:

$ ls -l /usr/bin/python3
lrwxrwxrwx 1 root root 24 Dec 31 16:17 /usr/bin/python3 -> /etc/alternatives/python

and in '/etc/alternatives' a link, named python which points to:

python -> /usr/local/bin/python3.9

or

python -> /usr/bin/python3.10

NOTE: You need to do the same for pip (or pip3 ), and other tools that relate to python3