Running Multiple Versions of Python on a Single Linux System

While some Python applications are written in Python 3, some others are still being maintained and developed in Python 2. That is to say, there is a need to run multiple versions of Python on a single system. The following steps show that you can install multiple versions of Python on a single Linux system, and activate and deactivate the version you wish to use conveniently by leveraging on virtual python environments. Note that the following steps are tested on a Debian-based Linux system, such as Debian Linux and Ubuntu Linux

1. Install the Python 2 and Python 3 if you have not done so.

   
   sudo apt-get install python python3
   

   
   
2. Install python-virtualenv packages. This step is the key.
   

   
   sudo apt-get install python-virtualenv
   

   
   
3. Show that you have both python 2 and 3 binaries. In the Linux system I am using for this post, the python binary by default points to Python 2's binary. Yours may differ.
   

   
   $ python --version
   Python 2.7.6
   
   $ python3 --version
   Python 3.4.3
   

   
   
4. To develop or run some applications in python 3, we need to activate Python 3. Then, first create a directory as the working directory for the Python 3 application. For instance, we name the directory as proj_python_3.
   

   
   $ mkdir proj_python_3
   

   
   
5. Now switch to the directory.
   

   
   cd proj_python_3
   
   $ which python3
   /usr/bin/python3
   

   
   
6. Then activate Python 3, we first need to set up an environment in the working directory for Python 3.
   

   
   $ virtualenv -p /usr/bin/python3 .env
   Running virtualenv with interpreter /usr/bin/python3
   Using base prefix '/usr'
   New python executable in .env/bin/python3
   Also creating executable in .env/bin/python
   Installing setuptools, pip...done.
   

   
   
7. Now we can activate Python 3 any time we wish after we switch to the working directory.
   

   
   $ pwd
   /home/user/proj_python_3
   
   $ . .env/bin/activate
   (.env)$
   

   
   Note that the prompt shows that you are under a virtual environment for Python 3. You can show that the activated Python version is Python 3.
   

   
   (.env)$ python --version
   Python 3.4.3
   

   
   
8. To deactivate the virtual environment, run the following,
   

   
   
   (.env)$ deactivate
   $ python --version
   Python 2.7.6
   

   
   

At present, the working directory for the virtual python environment must be on a file system that supports symbolic links; otherwise step 6 will fail.