How to Deploy Flask to Heroku: Step-by-Step Guide

To deploy a Flask app to Heroku, create a Procfile to tell Heroku how to run your app, add a requirements.txt with dependencies, and use Git to push your code to Heroku. Heroku will build and run your Flask app automatically after you push.

Syntax

Deploying Flask to Heroku involves these key files and commands:

• Procfile: Defines the command to start your app, e.g., web: gunicorn app:app.
• requirements.txt: Lists Python packages your app needs.
• runtime.txt (optional): Specifies Python version.
• Git commands: git init, heroku create, git push heroku main to deploy.

plaintext

Procfile
web: gunicorn app:app

requirements.txt
Flask
gunicorn

runtime.txt
python-3.11.4

Example

This example shows a simple Flask app deployed to Heroku. It includes the app code, necessary files, and deployment commands.

python

# app.py
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello, Heroku!'

# Procfile
web: gunicorn app:app

# requirements.txt
Flask
gunicorn

# Deployment commands
# git init
# heroku create
# git add .
# git commit -m "Initial commit"
# git push heroku main

Output

When visiting the Heroku app URL, the browser shows: Hello, Heroku!

Common Pitfalls

Common mistakes when deploying Flask to Heroku include:

• Not creating a Procfile, so Heroku doesn't know how to start the app.
• Forgetting to add gunicorn to requirements.txt, causing runtime errors.
• Using flask run in Procfile instead of gunicorn, which is not suitable for production.
• Not committing all files before pushing to Heroku.

plaintext

Wrong Procfile:
web: flask run

Right Procfile:
web: gunicorn app:app

Quick Reference

Step	Command/File	Purpose
1	git init	Initialize Git repository
2	heroku create	Create Heroku app
3	Procfile	Define app start command
4	requirements.txt	List Python dependencies
5	git add . && git commit -m 'msg'	Commit changes
6	git push heroku main	Deploy app to Heroku
7	heroku open	Open app in browser

Key Takeaways

Always create a Procfile with 'web: gunicorn app:app' to tell Heroku how to run your Flask app.

Include gunicorn in requirements.txt for production-ready server support.

Use Git to push your code to Heroku after setting up the app and files.

Avoid using 'flask run' in production; gunicorn is the recommended server.

Check that all necessary files are committed before deploying.