Saturday, August 12, 2017

Migration to Python 3 and virtualenv

So far Python Power Electronics has been built on Python2. I have Python 2.7.9 on computer and most of the other versions of Python I have used are quite similar i.e Python 2.7.x. Python 2 is now legacy and support will be disabled from 2020. Though that is a couple of years away, I figured I should start migrating since Python 3 is now well established and almost every OS ships with Python 3 by default and it may be soon when installing Python 2 may actually become an issue.

To start this migration, I don't want to do a system wide installation of Python 3 as it will break all my projects. So I am starting with virtualenv. To begin with, I am installing Python 3.5.4 into a separate folder.

In my Linux system, the system wide packages are installed in /usr. So I am installing Python 3.5.4 in home:
In the above user is the name of the user that you are logged in as. For that matter, you could install this in any other folder of your choice. I would strongly suggest that you install Python 3.5.4 in a user writable directory and not in a root directory.

So, download Python 3.5.4 from the Python website:

So let us suppose, this is downloaded to:
Extract the zip file to:
Change into this directory. In this directory you will find the entire source.

There will be some dependency issues - something is missing etc. I had a problem with libssl-dev being missing. I installed it using apt-get on my Debian system.

Anyway, first compile the source:
/home/user/Python-3.5.4/ $ ./confiure --prefix=/home/user/python_3_5_4

The prefix states that Python should be installed in a special folder and not in /usr/local/ which is the default.
If you get any errors and you might, Google each error and check which package is needed. Quite often it may say a package is missing when it is installed. What might actually missing is the development package. For example libssl may be installed, but libssl-dev may be missing. But that may cause it to exit with an error.

After that run the command:
/home/user/Python-3.5.4/ $ make

And if no errors:
/home/user/Python-3.5.4/ $  make install

The last command will create within /home/user/python_3_5_4 separate bin/, lib/ etc directories where Python will be installed.

Now, if you check:
python -V
You should get Python 2.7.x.

But if you run:
/home/user/python_3_5_4/bin/python3 -V
You should get Python 3.5.4.

So this means Python 3.5.4 has been installed in a separate directory. Now to begin using it.

Create a directory in home or anywhere else which will be the separate environment for Python 3.5.4. For example,

Change into this directory. Now you need virtualenv installed. I didn't have it so I used apt-get again to install virtualenv.

Run the command:
/home/user/python_3_5_4_virtual $ virtualenv -p /home/user/python_3_5_4/bin/python3 python3_install

What this will do it create a virtual environment but will use this Python 3.5.4 installed in an isolated directory to create an install environment in the directory called python3_install. So python3_install will contain all software related to Python - django, matplotlib, numpy, scipy etc.

Once this is done, activate it:
/home/user/python_3_5_4_virtual $ source python3_install/bin/activate

Now a special session will start. If within this session, you run:
(python3_install) /home/user/python_3_5_4_virtual $ python -V
You will get Python 3.5.4. The (python3_install) in bracket means you are in a virtual environment.

If you exit this environment by running:
(python3_install) /home/user/python_3_5_4_virtual $ deactivate

You exit. Which means:
/home/user/python_3_5_4_virtual $ python -V
Gives Python 2.7.9. Notice that (python3_install) has disappeared meaning you have exited the virtual environment.

So, to perform the migration, I will enter this virtual environment, install dependencies with Python 3.5.4 as the base. I will check the operation of the simulator bit by bit.