Setup a development environment

As an exercise, let us build a Python web application using the Flask web framework.

Create a new file default.nix. This file is conventionally used for specifying packages:

{ pkgs ? import <nixpkgs> {} }:

pkgs.python3Packages.buildPythonApplication {
  pname = "myapp";
  src = ./.;
  version = "0.1";
  propagatedBuildInputs = [ pkgs.python3Packages.flask ];
}

You will also need a simple Flask app as myapp.py:

#! /usr/bin/env python

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello, Nix!"

def run():
    app.run(host="0.0.0.0")

if __name__ == "__main__":
    run()

and a setup.py script:

from setuptools import setup

setup(
    name='myapp',
    version='0.1',
    py_modules=['myapp'],
    entry_points={
        'console_scripts': ['myapp = myapp:run']
    },
)

Now build the package with:

nix-build

This will create a symbolic link result to our package’s path in the Nix store, which looks like /nix/store/6i4l781jwk5vbia8as32637207kgkllj-myapp-0.1. Look around to see what is inside.

You may notice we can run the application from the package like ./result/bin/myapp.py. We can still use the default.nix as a shell environment to get the same result:

nix-shell default.nix
python3 myapp.py

In this context, Nix takes on the role that you would otherwise use pip or virtualenv for. Nix installs required dependencies and separates the environment from others on your system.

You can check this Nix configuration into version control and share it with others to make sure you are all running the same software. Especially with many dependencies this is a great way to prevent configuration drift between different team members & contributors.