Flask web server on Raspberry Pi

Introduction

A Flask web server lets your Raspberry Pi show web pages and respond to internet requests. It helps you create simple websites or control devices remotely.

You want to control Raspberry Pi devices from a phone or computer.

You want to create a small website hosted on your Raspberry Pi.

You want to collect data from sensors and show it on a webpage.

You want to learn how web servers work using a simple tool.

You want to build a home automation system accessible via browser.

Syntax

Raspberry Pi

from flask import Flask

app = Flask(__name__)

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

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

@app.route('/') tells Flask what web address to listen to.

app.run(host='0.0.0.0') makes the server visible to other devices on the network.

Examples

Basic Flask server running on default localhost and port 5000.

Raspberry Pi

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Welcome to my Pi!'

if __name__ == '__main__':
    app.run()

Server listens on all network interfaces on port 8080 with a custom route.

Raspberry Pi

from flask import Flask

app = Flask(__name__)

@app.route('/status')
def status():
    return 'Pi is running smoothly!'

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

Sample Program

This program starts a Flask web server on your Raspberry Pi. When you open your Pi's IP address in a browser, it shows 'Hello, Raspberry Pi!'.

Raspberry Pi

from flask import Flask

app = Flask(__name__)

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

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

Important Notes

Make sure Flask is installed on your Raspberry Pi using pip install flask.

Use host='0.0.0.0' to allow other devices on your network to access the server.

For security, this simple server is for learning and local use only, not for public internet.

Summary

Flask lets your Raspberry Pi serve web pages easily.

Use @app.route to set web addresses.

Run the server with app.run(host='0.0.0.0') to share on your network.

Practice

1. What does the @app.route decorator do in a Flask web server on Raspberry Pi?

easy

A. It installs Flask on the Raspberry Pi.

B. It starts the Flask server to listen for requests.

C. It defines the URL path that triggers a specific function.

D. It stops the Flask server from running.

Solution

1. Step 1: Understand the role of @app.route

   This decorator tells Flask which URL path should run the function below it.

2. Step 2: Connect URL to function

   When a user visits that URL, Flask runs the linked function to generate a response.

3. Final Answer:

   It defines the URL path that triggers a specific function. -> Option C

4. Quick Check:

   @app.route sets URL path [OK]

Hint: Remember: route = URL path to function [OK]

Common Mistakes:

• Thinking it starts or stops the server
• Confusing it with installing Flask
• Assuming it runs the server automatically

2. Which of the following is the correct way to run a Flask app on Raspberry Pi so it is accessible from other devices on the network?

easy

A. app.run(host='0.0.0.0')

B. app.run(host='127.0.0.1')

C. app.run()

D. app.run(port=8080)

Solution

1. Step 1: Understand host parameter in app.run()

   By default, Flask listens only on localhost (127.0.0.1), meaning only the Pi itself can access it.

2. Step 2: Use host='0.0.0.0' to allow network access

   Setting host to '0.0.0.0' tells Flask to listen on all network interfaces, so other devices can connect.

3. Final Answer:

   app.run(host='0.0.0.0') -> Option A

4. Quick Check:

   host='0.0.0.0' allows network access [OK]

Hint: Use host='0.0.0.0' to share server on network [OK]

Common Mistakes:

• Using default host which blocks external access
• Confusing port with host
• Using localhost IP which restricts access

3. What will be the output when you visit http://raspberrypi.local/hello given this Flask code on Raspberry Pi?

from flask import Flask
app = Flask(__name__)

@app.route('/hello')
def hello():
    return 'Hello, Raspberry Pi!'

if __name__ == '__main__':
    app.run(host='0.0.0.0')

medium

A. Server does not start

B. 404 Not Found error

C. SyntaxError

D. Hello, Raspberry Pi!

Solution

1. Step 1: Check the route and function

   The route '/hello' is defined and returns the string 'Hello, Raspberry Pi!'.

2. Step 2: Accessing the URL

   Visiting /hello triggers the function and returns the string as the web page content.

3. Final Answer:

   Hello, Raspberry Pi! -> Option D

4. Quick Check:

   Route '/hello' returns greeting string [OK]

Hint: Match URL path to function return value [OK]

Common Mistakes:

• Expecting a 404 error if route is correct
• Thinking code has syntax errors
• Assuming server won't start with host='0.0.0.0'

4. Identify the error in this Flask code snippet intended to run on Raspberry Pi:

from flask import Flask
app = Flask(__name__)

@app.route('/status')
def status():
print('Server is running')

if __name__ == '__main__':
    app.run(host='0.0.0.0')

medium

A. Incorrect host value in app.run().

B. Missing indentation for the print statement inside the function.

C. Missing import statement for Flask.

D. Route decorator syntax is wrong.

Solution

1. Step 1: Check function body indentation

   The print statement inside the function must be indented to be part of the function block.

2. Step 2: Verify other parts

   Host value '0.0.0.0' is correct, Flask is imported, and route syntax is valid.

3. Final Answer:

   Missing indentation for the print statement inside the function. -> Option B

4. Quick Check:

   Python requires indentation inside functions [OK]

Hint: Indent function code properly in Python [OK]

Common Mistakes:

• Ignoring indentation errors
• Changing host unnecessarily
• Misreading route decorator syntax

5. You want your Flask app on Raspberry Pi to show different messages at / and /info. Which code correctly sets up these two routes?

hard

A. from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
return 'Welcome!'

@app.route('/info')
def info():
return 'Raspberry Pi Info'

app.run(host='0.0.0.0')

B. from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
return 'Welcome!'

@app.route('/')
def info():
return 'Raspberry Pi Info'

app.run(host='0.0.0.0')

C. from flask import Flask
app = Flask(__name__)

@app.route('/info')
def home():
return 'Welcome!'

@app.route('/')
def info():
return 'Raspberry Pi Info'

app.run(host='0.0.0.0')

D. from flask import Flask
app = Flask(__name__)

@app.route('/home')
def home():
return 'Welcome!'

@app.route('/info')
def info():
return 'Raspberry Pi Info'

app.run(host='0.0.0.0')

Solution

1. Step 1: Check route paths and functions

   from flask import Flask app = Flask(__name__) @app.route('/') def home(): return 'Welcome!' @app.route('/info') def info(): return 'Raspberry Pi Info' app.run(host='0.0.0.0') correctly uses '/' for home and '/info' for info, each with its own function returning the right message.

2. Step 2: Verify no route conflicts

   from flask import Flask app = Flask(__name__) @app.route('/') def home(): return 'Welcome!' @app.route('/') def info(): return 'Raspberry Pi Info' app.run(host='0.0.0.0') uses '/' twice, which causes conflict. from flask import Flask app = Flask(__name__) @app.route('/info') def home(): return 'Welcome!' @app.route('/') def info(): return 'Raspberry Pi Info' app.run(host='0.0.0.0') swaps routes and functions incorrectly. from flask import Flask app = Flask(__name__) @app.route('/home') def home(): return 'Welcome!' @app.route('/info') def info(): return 'Raspberry Pi Info' app.run(host='0.0.0.0') uses '/home' instead of '/'.

3. Final Answer:

   Option A correctly defines two distinct routes '/' and '/info'. -> Option A

4. Quick Check:

   Unique routes need unique paths [OK]

Hint: Use unique URLs for each route function [OK]

Common Mistakes:

• Defining two functions for the same route
• Swapping route paths and function names
• Using wrong URL paths like '/home' instead of '/'