Change the Python3 default version in Ubuntu

Question

I am using Ubuntu 16.04 LTS . I have python3 installed. There are two versions installed, python 3.4.3 and python 3.6 . Whenever I use python3 command, it takes python 3.4.3 by default. I want to use python 3.6 with python3.

python3 --version shows version 3.4.3

I am installing ansible which supports version > 3.5 . So, whenever, I type ansible in the terminal, it throws error because of python 3.4

sudo update-alternatives --config python3
update-alternatives: error: no alternatives for python3

Answer 1

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

Answer 2

You can achieve this by applying below simple steps -

1. Check python version on terminal: python --version

2. Execute this command to switch to python 3.6:

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

3. Check python version: python --version

4. Done.

Answer 3

if you have multiple version of python in your system. You just need to update the symbolic link of python inside /usr/bin/

root@irshad:/usr/bin# ls -lrth python*
lrwxrwxrwx 1 root root    9 Apr 16  2018 python -> python2.7
-rwxr-xr-x 1 root root 3.6M Nov 12  2018 python2.7
-rwxr-xr-x 2 root root 4.4M May  7 14:58 python3.6

In above example if you see the output of python --version you will get python2.7

Now update the python symlink using below command-

root@irshad:/usr/bin# unlink python
root@irshad:/usr/bin# ln -s /usr/bin/python3.6 python
root@irshad:/usr/bin# python --version
Python 3.6.8

Answer 4

Using these commands can help you:

1. check the version of python: ls /usr/bin/python*
2. alias: alias python='/usr/bin/pythonxx' (add this to . ~/.bashrc)
3. re-login or source . ~/.bashrc
4. check the python version again: python --version

Answer 5

First check that you have a python3.6 folder?

ls /usr/bin/python3.6

If you have "python3.6" folder, you are good to go. Now update-alternatives

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1

then update new config for python3

sudo update-alternatives --config python3

Finally, check default python3 version:

python3 --version

Answer 6

Create symlink for /usr/bin/python3. In my LinuxMint:

# ls -lh /usr/bin/python3 /usr/bin/python
lrwxrwxrwx 1 root root 9 ноя 24  2017 /usr/bin/python -> python2.7
lrwxrwxrwx 1 root root 9 сен  6  2017 /usr/bin/python3 -> python3.5

# mv /usr/bin/python /usr/bin/python.bak
# cp /usr/bin/python3 /usr/bin/python
# python --version
Python 3.5.2

Answer 7

An easy answer would be to add an alias for python3.6.

Just add this line in the file ~/.bashrc : alias python3="python3.6", then close your terminal and open a new one. Now when you type python3 xxx it gets translated to python3.6 xxx.

This solution fixes your problem without needing to tweak your system too heavily.

EDIT :

As Mikael Kjær pointed out, this is a misconfiguration of ansible with your system.

As seen here :

Set the ansible_python_interpreter configuration option to /usr/bin/python3. The ansible_python_interpreter configuration option is usually set per-host as an inventory variable associated with a host or group of hosts:

  # Example inventory that makes an alias for localhost that uses python3
  [py3-hosts]
  localhost-py3 ansible_host=localhost ansible_connection=local

  [py3-hosts:vars]
  ansible_python_interpreter=/usr/bin/python3

As seen here about the config file :

Changes can be made and used in a configuration file which will be processed in the following order:

* ANSIBLE_CONFIG (an environment variable)
* ansible.cfg (in the current directory)
* .ansible.cfg (in the home directory)
* /etc/ansible/ansible.cfg

Answer 8

update-alternatives is to change system symlinks to user-defined/admin-defined symlinks. If you have multiple versions of python3 installed in your system and want to control which python3 version to invoke when python3 is called. Do the following

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.4 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5 2

Run below command if you want to change priority in the future.

update-alternatives --config python3

Explanation:-

sudo update-alternatives --install <symlink_origin> <name_of_config> <symlink_destination> <priority>

You can go on change name_of_config to python4, but then you have to invoke update-alternatives --config with python4 to reconfigure.

Using this approach you are able to control system python version and python3 version separately.

Answer 9

As mentioned in this comment by Slion,sudo apt install python-is-python3 will do this for you in a reasonable way, currently IIUC by added an appropriate symlink.

Answer 10

You can change the simbolic link by ln -sf python3.6 python3 inside /usr/bin. With this when you call python3 it will execute python3.6