Unable to set default python version to python3 in ubuntu

I was trying to set default python version to python3 in Ubuntu 16.04. By default it is python2 (2.7). I followed below steps :

update-alternatives --remove python /usr/bin/python2
update-alternatives --install /usr/bin/python python /usr/bin/python3

but I'm getting the following error for the second statement,

rejeesh@rejeesh-Vostro-1015:~$ update-alternatives --install /usr/bin/python python /usr/bin/python3
update-alternatives: --install needs <link> <name> <path> <priority>

Use 'update-alternatives --help' for program usage information.   

I'm new to Ubuntu and Idon't know what I'm doing wrong.

Solution 1:

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.

Solution 2:

EDIT:

I wrote this when I was young and naive, update-alternatives is the better way to do this. See @Pardhu's answer.

Outdated answer:

Open your .bashrc file nano ~/.bashrc. Type alias python=python3 on to a new line at the top of the file then save the file with ctrl+o and close the file with ctrl+x. Then, back at your command line type source ~/.bashrc. Now your alias should be permanent.

Solution 3:

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 

Solution 4:

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.