replace
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python3.5 3
with
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python3.5 3
e.g. installing into /usr/local/bin instead of /usr/bin.
and ensure the /usr/local/bin is before /usr/bin in PATH.
i.e.
[bash:~] $ echo $PATH
/usr/local/bin:/usr/bin:/bin
Ensure this always is the case by adding
export PATH=/usr/local/bin:$PATH
to the end of your ~/.bashrc file. Prefixing the PATH environment variable with custom bin folder such as /usr/local/bin or /opt/<some install>/bin is generally recommended to ensure that customizations are found before the default system ones.
replace
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python3.5 3
with
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python3.5 3
e.g. installing into /usr/local/bin instead of /usr/bin.
and ensure the /usr/local/bin is before /usr/bin in PATH.
i.e.
[bash:~] $ echo $PATH
/usr/local/bin:/usr/bin:/bin
Ensure this always is the case by adding
export PATH=/usr/local/bin:$PATH
to the end of your ~/.bashrc file. Prefixing the PATH environment variable with custom bin folder such as /usr/local/bin or /opt/<some install>/bin is generally recommended to ensure that customizations are found before the default system ones.
Per Debian policy, python refers to Python 2 and python3 refers to Python 3. Don't try to change this system-wide or you are in for the sort of trouble you already discovered.
Virtual environments allow you to run an isolated Python installation with whatever version of Python and whatever libraries you need without messing with the system Python install.
With recent Python 3, venv is part of the standard library; with older versions, you might need to install python3-venv or a similar package.
$HOME~$ python --version
Python 2.7.11
$HOME~$ python3 -m venv myenv
... stuff happens ...
$HOME~$ . ./myenv/bin/activate
(myenv) $HOME~$ type python # "type" is preferred over which; see POSIX
python is /home/you/myenv/bin/python
(myenv) $HOME~$ python --version
Python 3.5.1
A common practice is to have a separate environment for each project you work on, anyway; but if you want this to look like it's effectively system-wide for your own login, you could add the activation stanza to your .profile or similar.
To solve the error: no alternatives for python3 error message open the terminal and run the following command:
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 2
sudo update-alternatives --config python3
DON'T upgrade the default python version of your system. You will break it. Instead, set up virtual environments to run specific Python versions.
You are definitely able to do an "n to n" link as you state, just add the python3 group symlink you created as a potential target to python:
alternatives --install /usr/bin/python python /usr/bin/python3 300
alternatives --set python /usr/bin/python3
Each --install command will add or update an alternative, which you can see with alternatives --display python
Try update-alternatives --config python and update the current selection using the arrow keys. This gives the option to set the default python version.
From the comment:
sudo update-alternatives --config python
Will show you an error:
update-alternatives: error: no alternatives for python3
You need to update your update-alternatives , then you will be able to set your default python version.
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.4 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 2
Then run :
sudo update-alternatives --config python
Set python3.6 as default.
Or use the following command to set python3.6 as default:
sudo update-alternatives --set python /usr/bin/python3.6
You can achieve this by applying below simple steps -
Check python version on terminal:
python --versionExecute this command to switch to python 3.6:
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 1Check python version:
python --versionDone.