Objective
This article explains how to switch between Python2 and Python3 on Debian 9 Stretch Linux
Operating System and Software Versions
- Operating System: - Debian 9 Stretch
Requirements
Privileged access to to your Debian Linux installation will be required.
Difficulty
EASY
Conventions
- # - requires given linux commands to be executed with root privileges either directly as a root user or by use of
sudo command - $ - requires given linux commands to be executed as a regular non-privileged user
Instructions
Debian 9 Stretch comes with two
2.7 and
3.5 python versions. If you have not installed any python package yet you can choose between both by simply installing an appropriate package:
PYTHON 2 INSTALLATION:
# apt install python
PYTHON 3 INSTALLATION:
# apt install python3
Check Default Python Version
To check a default python version simply run
python command and query its version:
$ python --version
Python 2.7.13
Install Python
Let's assume that no python interpreter is not yet installed on our system. Hence, we can start by installation of both python versions:
# python --version
-bash: python: command not found
# apt install python python3
After installation the Python version
2.7 is made default:
$ python --version
Python 2.7.13
Update Python ALternatives List
To perform a system-wide switch between default python versions use
update-alternatives command. At first the
update-alternatives command will complain that there are no python alternatives available:
# update-alternatives --list python
update-alternatives: error: no alternatives for python
To install Python alternatives, first list all available options:
$ ls /usr/bin/python*
/usr/bin/python /usr/bin/python2 /usr/bin/python2.7 /usr/bin/python3 /usr/bin/python3.5 /usr/bin/python3.5m /usr/bin/python3m
Next, update the Python alternatives list for each version you whish to use. In our case with stick with
/usr/bin/python2.7 and
/usr/bin/python3.5 versions:
# update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in auto mode
# update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
update-alternatives: using /usr/bin/python3.5 to provide /usr/bin/python (python) in auto mode
Please note that the integer number at the end of each command denotes a priority. Higher number means higher priority and as such the
/usr/bin/python3.5 version was set in Auto Mode to be a default if no other selection is selected. After executing both above commands your current default python version is
/usr/bin/python3.5 due to its higher priority (
2):
# python --version
Python 3.5.3
Switch Between Python Versions
Now, that we have updated list of Python alternatives to perform a switch between any python version is to run:
# update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 2 auto mode
1 /usr/bin/python2.7 1 manual mode
2 /usr/bin/python3.5 2 manual mode
Press <enter> to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in manual mode
and select an appropriate version using selction integer as shown above.
# python --version
Python 2.7.13
Local User Python Version
In case you need to only change a python version selectively on per user basis, you may try to edit user's
.bashrc file. For example to change to python version
3.5 execute the following
linux commands:
$ python --version
Python 2.7.13
$ echo 'alias python="/usr/bin/python3.5"' >> ~/.bashrc
$ . .bashrc
$ python --version
Python 3.5.3
