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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

sudo update-alternatives --config python3
update-alternatives: error: no alternatives for python3
8
  • What is the command used to execute python 3.6 ?
    – user264413
    Dec 13, 2017 at 9:22
  • 7
    What does your sudo update-alternatives --config python3 say? Dec 13, 2017 at 9:35
  • 2
    update-alternatives: error: no alternatives for python3
    – codeclue
    Dec 13, 2017 at 9:38
  • 2
    @Mikael Different Python versions are not alternatives on Ubuntu. The OS relies on a specific version (or versions) being installed.
    – wjandrea
    Jan 1, 2020 at 19:22
  • 2
    Just run: sudo apt install python-is-python3
    – Slion
    Aug 31, 2021 at 18:18

9 Answers 9

368

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
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  • 28
    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. May 30, 2019 at 8:22
  • 10
    Making python3 point to python3.8 with this method broke gnome-terminal for me. Oct 17, 2019 at 23:49
  • 12
    @Johny I just added alias python="python3.8" to ~/.bash_profile Nov 25, 2019 at 21:02
  • 8
    Don't do this. It will break apt like @Giacomo mentioned along with other OS components like do-release-upgrade. Simply use an alias instead.
    – wjandrea
    Jan 1, 2020 at 19:15
  • 2
    Works fine in Ubuntu 18.04 LTS without breaking APT. Note to those confused: if you are getting the error: no alternatives for python message, then you need to run the --install commands following first (replacing the python versions you want to use as appropriate, and the numbers at the end are just for order) then you run the --config command again and you will be asked which version you want to use by default.
    – tpartee
    Feb 25, 2020 at 6:50
65

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. Type in your root 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.
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  • Works fine on Ubuntu 18.04. so simple than other solutions. thanks a lot Aug 13, 2019 at 23:29
  • 2
    This worked: sudo update-alternatives --set python /usr/bin/python3.7 python --version
    – Sam-T
    Dec 23, 2019 at 0:28
  • Also works on Pop! OS
    – Francis
    Jan 18, 2020 at 3:27
  • 12
    What is the point of step 3?
    – goonerify
    Feb 1, 2020 at 18:17
  • 1
    @goonerify it's the second half of step 2 Mar 30, 2020 at 11:16
32

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
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  • 3
    best answer! thanks friend :)
    – Bryan_C
    Dec 18, 2020 at 3:52
  • 1
    the best way without disturbing anything on the ground. Jun 27, 2021 at 1:19
  • 1
    Best answer , this must be the accepted answer , tnx bro
    – ParSa
    Nov 25, 2021 at 13:16
  • 1
    I agree this answer is the best. However, I would add that some people's newly installed python won't be located in /usr/bin/pythonX.X. Instead figure out the location with which pythonX.X and substitute in. Jan 19 at 1:07
  • I couldn't find the newly installed python3.9 in /user/bin/, any suggestion please? I am on Ubuntu 18.04
    – Adamy
    Jun 17 at 0:37
22

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
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  • 1
    thanks but after restarting the computer the main python would be still the ex python 2.7 Aug 13, 2019 at 23:25
  • 3
    You should add the script to modify the default in ~/.bashrc. Then you will make it work forever.
    – Newt
    Nov 4, 2019 at 13:32
  • like this: export python=/home/xxx (your py3 directory)
    – Newt
    Nov 4, 2019 at 13:40
13

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
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  • 1
    nit: you don't have folders under bin, you have executables.
    – mayid
    Oct 1, 2020 at 22:52
  • Didn't work for me.
    – Evan Zamir
    Sep 6, 2021 at 0:15
  • 1
    This broke sudo apt-get update for me ModuleNotFoundError: No module named 'apt_pkg'
    – Mark
    Dec 7, 2021 at 22:51
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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
1
  • 3
    This broke apt installer.
    – jws
    May 5, 2020 at 21:30
5

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
3
3

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
  • This is one of only 2 answers that updates python3 without updating the python version and this one seems to have been first. Thanks!!
    – sage
    Aug 12, 2021 at 18:39
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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