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Various programming stuff

Using both Python 2 and 3 in Windows

The release of Django 2.0 was a milestone in the history of Python since it completely dropped support for Python 2.x. I am a long time user of Django and, loyal to the philosophy of “if it is working don’t change” I was always using Python 2.x.

Of course right now this needs to change since new applications will need to be developed to the latest version of Django (and Python). I am using Windows for development (almost exclusively) and what I wanted was to be able to create and use virtual environments for both my old projects (using Python 2) and new projects (using Python 3).

The above requirement is not as straight-forward as I’d like - actually it is if you know what you need to do and which tools you must use. That’s why I decided to write a quick step by step tutorial on how to use both versions of Python in your Windows environment. Notice that I am using Windows 10, Python 2.7.14 an Python 3.6.4.

First of all, let’s download both versions of Python from the Python download page. I downloaded the files python-2.7.14.msi and python-3.6.4.exe (not sure why the one is .msi and the other is .exe it doesn’t matter anyway).

Firstly I am installing Python 2.7.14 and selecting:

  • Install for all users
  • Install to default folder
  • Press next to following screen (install default customizations)
  • This won’t add the python.exe of Python 2.7 to path

Next I am install Python 3.6.4:

  • Make sure to click “Install launcher for all users (recommended)”
  • I also check “Add Python 3.6to PATH” (to add the Python 3.6 executable to path)
  • I then just click “Install Now” (this will put Python 3.6 to c:)

Right now if you open a terminal window (Windows+r, cmd.exe) and run Python you will initiate the Python 3.6 interpreter. This is useful for just dropping in a Python interpreter.

Remember the “Install launcher for all users (recommended)” we clicked before? This installs the python launcher which is used to select between Python versions. If you run it without parameters the Python 3.6 interpreter will by started. You can pass the -2 parameter to start the python 2.x interpreter or -3 to explicitly declare the python 3.x interpreter:

C:\Users\Serafeim>py -2
Python 2.7.14 (v2.7.14:84471935ed, Sep 16 2017, 20:19:30) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> ^Z


C:\Users\Serafeim>py -3
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> ^Z

With py we can easily start the Python interpreter we want. This is not enough though - we need to use virtualenv and create proper virtual environments for our projects. To do this you can add the -m option to py to run a module with the proper python version. For example, to start an http server with Python 2 you would use the module SimpleHTTPServer while for python 3 you would use http.server (as per this):

C:\progr\py\pelican\spapas.github.io>py -2 -m SimpleHTTPServer
Serving HTTP on 0.0.0.0 port 8000 ...

C:\progr\py\pelican\spapas.github.io>py -3 -m http.server
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...

Now, to create the virtual environments we’ll use the virtualenv module which is installed by default in Python 2.x (try running py -2 -m pip freeze to see the list of installed packages for Python 2.x) and the venv module which is included in the Python 3.x core. So to create a virtualenv for Python 2 we’ll run py -2 -m virtualenv name-of-virtualenv and for Python 3 py -3 -m venv name-of-virtualenv.

C:\progr\py>py -2 -m virtualenv venv-2
New python executable in C:\progr\py\venv-2\Scripts\python.exe
Installing setuptools, pip, wheel...done.

C:\progr\py>py -3 -m venv venv-3

C:\progr\py>venv-2\Scripts\activate

(venv-2) C:\progr\py>python -V
Python 2.7.14

(venv-2) C:\progr\py>deactivate

C:\progr\py>venv-3\Scripts\activate

(venv-3) C:\progr\py>python -V
Python 3.6.4

That’s how easy it is to have both Python 2.7 and Python 3.6 in your Windows!

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