Environment Variables

In this article, you'll learn how to configure a Python web application using environment variables. We’ll start by examining a basic Flask application that renders a web page with a background color. Initially, the background color is hardcoded in the application source code.

Below is the initial code snippet where the background color is set to "red":

import os
from flask import Flask, render_template

app = Flask(__name__)
color = "red"

@app.route("/")
def main():
    print(color)
    return render_template('hello.html', color=color)

if __name__ == "__main__":
    app.run(host="0.0.0.0", port="8080")

In this version, any change to the background color requires direct modification of the source code. Best practices suggest externalizing configuration data, such as the background color, to keep your application logic independent from configuration changes.

To implement this, we introduce an environment variable named APP_COLOR. By reading the value of APP_COLOR from the environment, you can easily update the background color without modifying the application's source code. Below is the updated version of the code:

import os
from flask import Flask, render_template

app = Flask(__name__)
color = os.environ.get('APP_COLOR')

@app.route("/")
def main():
    print(color)
    return render_template('hello.html', color=color)

if __name__ == "__main__":
    app.run(host="0.0.0.0", port="8080")

Running the Application with an Environment Variable

You can start the application with the desired background color by setting the environment variable inline. For example, to run the app with a blue background, use the following command:

export APP_COLOR=blue; python app.py

Deploying with Docker

When deploying your application as a Docker container, you can pass the environment variable at runtime using the Docker run command. For example, to run the container with the background color set to blue, execute:

docker run -e APP_COLOR=blue simple-webapp-color

To deploy multiple containers with different background colors, you can run multiple containers with distinct values:

docker run -e APP_COLOR=blue simple-webapp-color
docker run -e APP_COLOR=green simple-webapp-color
docker run -e APP_COLOR=pink simple-webapp-color

Inspecting Environment Variables in a Running Container

To verify the environment variables set within a running container, use the Docker inspect command. The output will list the environment variables under the "Config" section. For example:

docker inspect blissful_hopper

The output includes a section similar to the following:

[
  {
    "Id": "35505f7810d17291261a43391d4b6c0846594d415ce4f4d0a6ffbf9cc5109048",
    "State": {
      "Status": "running",
      "Running": true
    },
    "Mounts": [],
    "Config": {
      "Env": [
        "APP_COLOR=blue",
        "LANG=C.UTF-8",
        "GPG_KEY=0D96DF4D4110E5C43FBFB17F2D347EA6AA65421D",
        "PYTHON_VERSION=3.6.6",
        "PYTHON_PIP_VERSION=18.1"
      ],
      "Entrypoint": [
        "python",
        "app.py"
      ]
    }
  }
]

This section demonstrates how the APP_COLOR environment variable is configured in the container along with other system-level environment settings.

That’s it for this article on using environment variables in Python web applications and Docker deployments for managing application settings.