Update alternatives error no alternatives for python3

On our Kali Linux (or any other Linux distribution) we might have installed different versions of Python. For using Python version 2.x we generally use python2 command, same as for using Python 3.x versions we use python3 command.

Here assume that we have installed multiple versions of Python3 installed on our system, like we have installed Python3.7 and Python 3.9 both on our Linux system for any reason. So whenever we want to use Python 3.9 we need to type command python3.9 because python3 command using Python 3.7 version as default.

Our advanced Linux users may know this problem and the solution, but this is for beginners.

How to check installed Python versions on Linux?

This can be easily done with a simple command on our Terminal window. The command is following:

ls /usr/bin/python*

In the following screenshot we can see that we have Python2.7, Python3.7 and Python3.9 installed on our system.

Problem

But we can see that python3 command is choosing Python3.7 version as default. But some updated tools needs Python3.9 to run. We can run python3.9 command, but it is annoying we should run python3 to run Python3 latest version, we may modify our .bashrc/.zshrc file but that will not be the correct solution.

We need to set our update-alternatives for python3.

We can check for the alternatives of python3 by running following command:

sudo update-alternatives --config python3

But here we might get an error «update-alternatives: error: no alternatives for python3«.

It means, first we need to set alternatives for python3.

Solved

To set the alternatives for python3 we need to run some commands on our terminal.

First of all we need to run the following command:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1

This command will add Python 3.7 on option 1.

Then we need to run following command:

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

This command will add Python 3.9 on option 2

We can see this on the following screenshot:

Now we can again run the configure command to check and set the alternatives:

sudo update-alternatives --config python3

In the following screenshot we can see that now we can save the configurations now.

Here we can set the default version for the python3. Here automatically 0 is chosen for Python 3.9 version, we can go for it, otherwise instead of choosing by numbers we can run following command to choose the default python3 version:

sudo update-alternatives  --set python3 /usr/bin/python3.9

Now we can check python3 default version by using following command:

python3 -V

We can see that now our Python 3.9 version is set as default for python3 command:

«update-alternatives: error: no alternatives for python3» is a very common problem for beginners so we thought to write an entire article for it we got too much request to solve this on our Telegram DM. When Python 4 will release some versions of Python 4, we can use the same as we did for Python 3.

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The other day I decided that I wanted the command python to default to firing up python3 instead of python2.

So I did this:

$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 2

$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 3

$ sudo update-alternatives --config python

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

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.5   3         auto mode
  1            /usr/bin/python2.7   2         manual mode
  2            /usr/bin/python3.5   3         manual mode

Press <enter> to keep the current choice[*], or type selection number: 0

And that all worked. Great!

$ python -V
Python 3.5.2

But it wasn’t long before I realised I had broken apt/aptitude when it comes to installing and removing python packages because apt was expecting python2 it transpired.

This is what happened.

$ sudo apt remove  python-samba
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n] 
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...
  File "/usr/bin/pyclean", line 63
    except (IOError, OSError), e:
                             ^
SyntaxError: invalid syntax
dpkg: error processing package python-samba (--remove):
 subprocess installed pre-removal script returned error exit status 1
Traceback (most recent call last):
  File "/usr/bin/pycompile", line 35, in <module>
    from debpython.version import SUPPORTED, debsorted, vrepr, 
  File "/usr/share/python/debpython/version.py", line 24, in <module>
    from ConfigParser import SafeConfigParser
ImportError: No module named 'ConfigParser'
dpkg: error while cleaning up:
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 python-samba
E: Sub-process /usr/bin/dpkg returned an error code (1)

Eventually I guessed it wanted python2 as the default, so I undid my changes as follows:

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

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.5   3         auto mode
  1            /usr/bin/python2.7   2         manual mode
  2            /usr/bin/python3.5   3         manual mode

Press <enter> to keep the current choice[*], or type selection number: 1

$ python -V
Python 2.7.12

And then apt worked again

$ sudo apt remove  python-samba
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n] 
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...

So I have had to leave it as defaulting to python 2 but I develop in python 3 and so would like my system to default to python 3 for when I run python and idle.

Can anyone tell me how I can achieve this without breaking apt?

My system is a Raspberry Pi 3B running Ubuntu:

Linux mymachine 4.4.38-v7+ #938 SMP Thu Dec 15 15:22:21 GMT 2016 armv7l armv7l armv7l GNU/Linux

(It’s actually an arm v8)

$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.2 LTS"

Learn how to use the PPA repository to install Python’s old versions such as 3.9. 3.8, 3.7, and more on Ubuntu 22.04 Jammy JellyFish using the command terminal. 

Python is freely available for the common operating systems. The programming language is standard equipment in many Linux distributions. Python can also be used on many mobile operating systems. For web servers, WSGI (Web Server Gateway Interface) is a universal interface between the server and Python.

It offers clear syntax and good readability. It is considered easy to learn and can be interpreted in the common operating systems. Python supports several programming paradigms such as functional, object-oriented, or aspect-oriented programming and can also be used as a scripting language.

Since Python is a so-called multiparadigm language, programmers are not tied to a specific programming style. The optimal programming style can be selected for the various tasks. Python allows Python programs to be embedded in other languages ​​as individual modules.

There are many scenarios when we especially the developers need to have an old python version to work with some particular application or on the project. In this tutorial, we learn how to get that…

1. Start with the system update

Execute the system update command to ensure our Ubuntu 22.04 has the latest security and package versions. Open your terminal and run:

sudo apt update && sudo apt upgrade

2. Add PPA for Python old versions

The old versions of Python such as 3.9, 3.8, 3.7, and older are not available to install using the default system repository of Ubuntu 22.04 LTS Jammy JellyFish or 20.04 Focal Fossa. Hence, we need to add a PPA offered by the “deadsnakes” team to get the old archived Python versions easily.

sudo apt install software-properties-common

sudo add-apt-repository ppa:deadsnakes/ppa

3. Check Python Versions you want

Once you have added the PPA repository, we can install most of the old python versions with just one command. To confirm the version you want is available to install or not, you can run:

sudo apt-cache policy python(version)

Example:

sudo apt-cache policy python3.9

4. Install Python 3.9 on Ubuntu 22.04

Well, once you have confirmed the version you want of Python is available to install using the added repository, next use the given command syntax to install it.

For example, you want to install version 3.9

sudo apt install python3.9

5. Install Python 3.8 on Ubuntu

The way we have installed version 3.9 using the PPA, we can go for the 3.8 as well.

sudo apt install python3.8

In the same way, we can install Python other versions such as 3.7, 3.6, and more…

6. Set the default Python version

If you have installed multiple versions of Python and want to set any of them as the default version of the system then that can be possible as well. For example, you have the latest version of Python 3.10 (was while doing this article) and also have installed python 3.9 and 3.8; now to set any of them are system’s default use Update-alternatives command:

Check what python versions are available on your systems:

ls /usr/bin/python*

To know whether any version is configured as python alternatives or not. For that run:

sudo update-alternatives --list python

If the output is:

“update-alternatives: error: no alternatives for python”

Then it means there are no alternatives that have been configured, hence let’s do some.

Here we are setting up two versions as alternatives, later we will be able to choose between them to set one of them as the system default one.

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.10 1

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.9 2

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.8 3

Switch the default Python version 

sudo update-alternatives --config python

This time you will see several options including Python 3.9 and 3.8. Enter the Selection number of the Python you want to set as the system’s default one. For example, here Python3.9 is at number 3, hence we type that and Enter key.

When you check the version of Python this time, you will have 3.9. For that use:

python -V

7. Uninstall Python and PPA

Those who don’t want to the installed python version on the system and also want to remove the PPA if doesn’t require any more, use the given commands.

sudo apt remove --purge python3.9

In the same way for 3.8:

sudo apt remove --purge python3.8

To remove PPA:

sudo add-apt-repository --remove ppa:deadsnakes/ppa

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