Environment Variables

This guide shows a simple, configurable Flask web application that reads its background color from an environment variable instead of hard-coding it. Using environment variables for configuration keeps deployment and application concerns separated and makes it easy to change behavior across environments without modifying code. The sample app is based on the Python Basics course.

Hard-coded color (initial example)

A typical Flask app that hard-codes the color looks like this:

import os
from flask import Flask, render_template

app = Flask(__name__)

@app.route("/")
def index():
    color = "red"  # hard-coded color
    return render_template("index.html", color=color)

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

Hard-coding values like this requires changing application code whenever you change configuration. Moving configuration into environment variables allows changing behavior at runtime or deployment time instead.

Read color from an environment variable

Update the app to read from an environment variable named APP_COLOR, with a sensible default:

import os
from flask import Flask, render_template

app = Flask(__name__)

@app.route("/")
def index():
    # Read color from environment, default to "red" if not set
    color = os.environ.get("APP_COLOR", "red")
    return render_template("index.html", color=color)

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

This small change decouples configuration from code and works the same in local development and inside containers.

Run locally with different colors

You can set the environment variable before running, or inline for a single command:

# Option 1: export then run
export APP_COLOR=blue
python app.py

# Option 2: inline for a single command
APP_COLOR=blue python app.py

Build and run in Docker

When you package the application into a Docker image (for example, simple-webapp-color), pass the environment variable into the container at runtime using -e:

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

To run multiple containers with different colors, start multiple containers and provide a different APP_COLOR value for each docker run invocation.

Quick reference: common commands

Purpose	Command / Example
Run locally with exported env var	`export APP_COLOR=blue` then `python app.py`
Run locally inline	`APP_COLOR=blue python app.py`
Run container with env var	`docker run -e APP_COLOR=blue simple-webapp-color`
Inspect container env vars (compact)	`docker inspect -f '{{json .Config.Env}}' <container-id>`

Using environment variables for configuration keeps code and deployment concerns separate and makes it easy to change behavior across environments without modifying the application.

Inspecting environment variables in a container

To see which environment variables are set inside a running container, use docker inspect. The container properties include a Config.Env section that lists environment variables. A full docker inspect <container-id> will include JSON like this:

[
  {
    "Id": "35505f7810d17291261a43391d4b6c0846594d415ce4f4d0a6ffbf9cc5109048",
    "State": {
      "Status": "running",
      "Running": true
    },
    "Mounts": [],
    "Config": {
      "Env": [
        "APP_COLOR=blue",
        "LANG=C.UTF-8"
      ]
    }
  }
]

For a compact view of only the environment variables, use:

docker inspect -f '{{json .Config.Env}}' <container-id>

Note: wrap the template {{json .Config.Env}} in backticks when including it in MDX or other contexts that may interpret curly braces.

Why environment variables?

Simple and language-agnostic: Most runtimes expose environment variables easily.
Secure for secrets when used with secret managers (avoid committing secrets to source code).
Works well with containers and orchestration platforms (Docker, Kubernetes, etc.).
Allows different settings per environment without code changes.

Links and references

Python Basics
Docker Training Course for the Absolute Beginner
Docker documentation: https://docs.docker.com/

Introduction

Docker Commands

Docker Images

Docker Compose

Docker Engine, Storage

Docker Networking

Docker Registry

Docker on Mac & Windows

Container Orchestration - Docker Swarm & Kubernetes

Conclusion

Docker Run Commands

Environment Variables

Hard-coded color (initial example)

Read color from an environment variable

Run locally with different colors

Build and run in Docker

Quick reference: common commands

Inspecting environment variables in a container

Why environment variables?

Links and references

Watch Video

Practice Lab

​Hard-coded color (initial example)

​Read color from an environment variable

​Run locally with different colors

​Build and run in Docker

​Quick reference: common commands

​Inspecting environment variables in a container

​Why environment variables?

​Links and references

Watch Video

Practice Lab

Hard-coded color (initial example)

Read color from an environment variable

Run locally with different colors

Build and run in Docker

Quick reference: common commands

Inspecting environment variables in a container

Why environment variables?

Links and references