fredag 15 mars 2019

Pythons & Windows

For a couple of years now, I have been developing software on machines with Mac OS X or Linux. Installing dev environments and tools for coding in languages like Node.js or Python is usually without any issues. Or is it just luck? At work, our team maintain packages, apps, tools and frameworks written in different languages (JavaScript, C#, Python, Ruby). Everything we develop should be platform independent.

Recently, we built a tool written in Python 3 and the tool is intended to be used by many teams within the organization. So, I asked a fellow developer with a Windows machine to try it out. As it turned out, installing Python on Windows was ... a bit different. 

Why use more than one version of Python?
It's important for us to be able to run both Python 2.7 and 3 on the same machine, because of peer projects using build tools like node-gyp (that still has dependencies to Python 2).

This is what we learned about setting up and run both the Pythons on a Windows machine.

First step: Download and install Python
Download the latest version of Python from
Install Python 3.7.2 or later using the downloaded installer.

What about Python 2.7?
Ok, Python 2.7 was already installed on the Windows machines we used, so we skipped that step. But you can download and install it from the same site, or install the windows-build-tools.

Use PowerShell
PowerShell is cool. To make PowerShell as smooth as Bash, I think you should create aliases for the different Python versions. An alias can be added to the PowerShell profile. Check out the guide from Microsoft about verifying existing profiles and creating new ones.

Adding Python aliases to your PowerShell profile will make it possible to run both Python 2.7 and Python 3 in the same way as in other systems.

But where did the Windows installers put the Pythons? To find out, you can run this command in PowerShell:
where.exe python

Now you're ready to edit the PowerShell profile. Open it by typing notepad $profile, and add these rows:
New-Alias python C:\<my path to Python 2.7>\python.exe
New-Alias python3 C:\<my path to Python 3.7.2 or later>\python.exe 

Note: you may need an additional step, enabling profile scripts to run when a PowerShell window is opened. To do that, open a new PowerShell window in Admin mode (important) and write:
Set-ExecutionPolicy RemoteSigned

When finished, close the admin window, and open a new regular one. 

Almost done 
Verify the aliases, by typing:

python --version
(should output something like 2.7.*)

python3 --version 
(should output something like 3.7.*)

The Python virtual environment on Windows
Creating a virtual environment is the same as in Mac OS X or Linux: Install it with pip install virtualenv (only once), and create a virtual environment in your Python project folder with virtualenv venv.

To activate in bash, I'm used to write source venv/bin/activate

But that doesn't work on Windows. Instead, VirtualEnv for Windows have custom PowerShell scripts prepared for activation. You'll find them in the venv\Scripts folder that virtualenv has created with the virtualenv venv command.

.\venv\Scripts\activate.ps1 will activate the virtual environment in your Python folder.

With this setup, running Python apps will be close to identical to the setups in Mac OS X and Linux.

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