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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

  • What is the command used to execute python 3.6 ? – user264413 Dec 13 '17 at 9:22
  • command is :- python3.6 – codeclue Dec 13 '17 at 9:23
  • 2
    What does your sudo update-alternatives --config python3 say? – Mikael Kjær Dec 13 '17 at 9:35
  • update-alternatives: error: no alternatives for python3 – codeclue Dec 13 '17 at 9:38
  • See my comment in the below answer on how to use ansible with a specific version of python3. You could also consider removing python 3.4. – Mikael Kjær Dec 13 '17 at 9:43
79

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
  • 1
    This answer will probably break apt. After selecting a different version of python try to run sudo apt update. If it gives you an error like ModuleNotFoundError: No module named 'apt_pkg' you want to go to /usr/lib/python3/dist-packages and create the symlink: ln -s apt_pkg.cpython-{34m,36m}-x86_64-linux-gnu.so. – Giacomo Alzetta May 30 at 8:22
  • It gives me an error: $ sudo update-alternatives --set python /usr/bin/python3.6 update-alternatives: error: no alternatives for python Any suggestions? – Slava Fomin II Sep 10 at 15:34
  • 1
    @GAD3R Thanks. I've managed to create a symlink manually to point to the Python 3.6. Also, I don't have 3.7 installed. – Slava Fomin II Sep 11 at 8:58
7

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
# cp /usr/bin/python3 /usr/bin/python
# python --version
Python 3.5.2
7

You can achieve this by applying below simple steps -

  1. Check python version on terminal - python --version
  2. Get root user privileges. On terminal type - sudo su
  3. Write down the root user password
  4. Execute this command to switch to python 3.6 -
    update-alternatives --install /usr/bin/python python /usr/bin/python3 1
  5. Check python version - python --version
  6. Done.
  • 1
    Works, is simple to read and does not use alias. +1 – Carlos Pinzón Jun 16 at 15:04
  • Works fine on Ubuntu 18.04. so simple than other solutions. thanks a lot – Mohammad Heydari Aug 13 at 23:29
6

using these commands can help you:

  1. check the version of python: ls /usr/bin/python*
  2. alias: alias python='/usr/bin/pythonxx'
  3. re-login: . ~/.bashrc
  4. check the python version again: python --version
  • thanks but after restarting the computer the main python would be still the ex python 2.7 – Mohammad Heydari Aug 13 at 23:25
3

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
1

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.

1

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
0

the first check do you python3.6 folder

ls /usr/bin/python3.6

if you have folder like python3.6 good to go

then update-alternatives

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

now update new config for python3

sudo update-alternatives --config python3

check python version

python3 --version

0

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

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