Unable to set default python version to python3 in ubuntu

To change to python3, you can use the following command in terminal alias python=python3.

As it says, update-alternatives --install needs <link> <name> <path> and <priority> arguments.

You have link (/usr/bin/python), name (python), and path (/usr/bin/python3), you’re missing priority.

update-alternatives --help says:

<priority> is an integer; options with higher numbers have higher priority in automatic mode.

So just put a 100 or something at the end

Update:
Since Ubuntu 20.04, the python3 is the default version, but still, python is not registered as python3 by default. In order to make that happen, you can simply do :

sudo apt install python-is-python3

For more information you can check this out.
Old way:

Do

cd ~
gedit .bash_aliases

then write either

alias python=python3

or

alias python='/usr/bin/python3'

Save the file, close the terminal and open it again.
You should be fine now! Link

A simple safe way would be to use an alias. Place this into ~/.bashrc file:
if you have gedit editor use

gedit ~/.bashrc

to go into the bashrc file and then at the top of the bashrc file make the following change.

alias python=python3

After adding the above in the file. run the below command

source ~/.bash_aliases or source ~/.bashrc

example:

$ python –version

Python 2.7.6

$ python3 –version

Python 3.4.3

$ alias python=python3

$ python –version

Python 3.4.3

As an added extra, you can add an alias for pip as well (in .bashrc or bash_aliases):

alias pip=’pip3′

You many find that a clean install of python3 actually points to python3.x so you may need:

alias pip=’pip3.6′
alias python=’python3.6′

The second line mentioned can be changed to

[sudo] update-alternatives --install /usr/bin/python python /usr/bin/python3 10

This gives a priority of 10 for the path of python3.

The disadvantage of alternatively editing .bashrc is that using the commands with sudo will not work.

get python path from

ls /usr/bin/python*

then set your python version

alias python="/usr/bin/python3"

For another non-invasive, current-user only approach:

# First, make $HOME/bin, which will be automatically added to user's PATH
mkdir -p ~/bin
# make link actual python binaries
ln -s $(which python3) python
ln -s $(which pip3) pip

python pip will be ready in a new shell.

The best way in ubuntu 18.04 which will work for all users is

sudo vim /etc/bash.bashrc
add lines
alias python=python3
alias pip=pip3

Save the changes and restart .

After restart what ever version of python 3 you have in the system along with python 2.7 will be taken as default. You could be more specific by saying the following in alias if you have multiple version of python 3.

sudo vim /etc/bash.bashrc
add lines
alias python=python3.6
alias pip=pip3.6

To change Python 3.6.8 as the default in Ubuntu 18.04 to Python 3.7.

Install Python 3.7

Steps to install Python3.7 and configure it as the default interpreter.

1. Install the python3.7 package using apt-get

   sudo apt-get install python3.7

2. Add Python3.6 & Python 3.7 to update-alternatives

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 2

3. Update Python 3 to point to Python 3.7

   sudo update-alternatives --config python3
   Enter 2 for Python 3.7

4. Test the version of python

python3 --version
Python 3.7.1 

At first, Make sure Python3 is installed on your computer

Go to your terminal and type:

cd ~/ to go to your home directory

If you didn’t set up your .bash_profile yet, type touch .bash_profile to create your .bash_profile.

Or, type open -e .bash_profile to edit the file.

Copy and save alias python=python3 in the .bash_profile.

Close and reopen your Terminal. Then type the following command to check if Python3 is your default version now:

python --version

You should see python 3.x.y is your default version.

Cheers!

At First Install python3 and pip3

sudo apt-get install python3 python3-pip

then in your terminal run

alias python=python3

Check the version of python in your machine.

python --version

Just follow these steps to help change the default python to the newly upgrade python version. Worked well for me.

• sudo apt-install python3.7 Install the latest version of python you want
• cd /usr/bin Enter the root directory where python is installed
• sudo unlink python or sudo unlink python3 . Unlink the current default python
• sudo ln -sv /usr/bin/python3.7 python Link the new downloaded python version
• python --version Check the new python version and you’re good to go

If you have Ubuntu 20.04 LTS (Focal Fossa) you can install python-is-python3:

sudo apt install python-is-python3

which replaces the symlink in /usr/bin/python to point to /usr/bin/python3.

Simply remove python-is-python2:

sudo apt purge python-is-python2

And install python-is-python3:

sudo apt install python-is-python3

It will automate the process of transition to new python3. Optionally you can get rid of remaining packages later:

sudo apt autoremove && sudo apt autoclean

To change Python 3.6.8 as the default in Ubuntu 18.04 from Python 2.7 you can try the command line tool update-alternatives.

sudo update-alternatives --config python

If you get the error "no alternatives for python" then set up an alternative yourself with the following command:

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

Change the path /usr/bin/python3 to your desired python version accordingly.

The last argument specified it priority means, if no manual alternative selection is made the alternative with the highest priority number will be set. In our case we have set a priority 2 for /usr/bin/python3.6.8 and as a result the /usr/bin/python3.6.8 was set as default python version automatically by update-alternatives command.

we can anytime switch between the above listed python alternative versions using below command and entering a selection number:

update-alternatives --config python

You didn’t include the priority argument

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 5

You can replace 5 with any priority you want. A higher priority alternative takes precedence over lower priority.

Set priority for default python in Linux terminal by adding this:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 10
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 1

Here, we set python3 to have priority 10 and python2 to priority 1. This will make python3 the default python. If you want Python2 as default then make a priority of python2 higher then python3

This is a simple way that works for me.

sudo ln -s /usr/bin/python3 /usr/bin/python

You could change /usr/bin/python3 for your path to python3 (or the version you want).

But keep in mind that update-alternatives is probably the best choice.

sudo rm /usr/bin/python3 #remove existing link
sudo ln /usr/bin/python3.8 /usr/bin/python3 # create a new link to the version of your choice

If there is a possibility to use particular python version directly, I would go for it compared to update-alternatives and alias solution.

Ex.

python3.6 -m pip install pytest
ptyhon3.6 -m pytest test_sample.py

-m executes particular module for that particular python version.
The first line will install pytest for for that particular version and user in possible location /home/user/.local/lib/python3.5/site-packages

       ~$ sudo apt-get install python3.9
/usr/bin$ cd /usr/bin
/usr/bin$ sudo unlink python3
/usr/bin$ sudo ln -sv /usr/bin/python3.9 python3
/usr/bin$ python3 --version
          Python 3.9.5
/usr/bin$ pip3 --version
          pip 20.0.2 from /usr/lib/python3/dist-packages/pip (python 3.9)

To change the default python version in linux use following command-

sudo ln -sf /usr/bin/python3 /usr/bin/python2

This will change default version to python3

To verify use command-

python --version