Flask web server on Raspberry Pi - Time & Space Complexity

Time Complexity: Flask web server on Raspberry Pi

O(n)

Understanding Time Complexity

When running a Flask web server on a Raspberry Pi, it is important to understand how the server handles requests as more users connect.

We want to know how the time to respond grows when more requests come in.

Scenario Under Consideration

Analyze the time complexity of the following Flask server code snippet.

from flask import Flask
app = Flask(__name__)

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

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

This code starts a simple Flask web server that responds with a greeting message to each request at the home page.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Handling each incoming web request one by one.
• How many times: Once per request, repeated as many times as users connect.

How Execution Grows With Input

Each new user request causes the server to run the handler function once.

Input Size (n requests)	Approx. Operations
10	10 handler calls
100	100 handler calls
1000	1000 handler calls

Pattern observation: The number of operations grows directly with the number of requests.

Final Time Complexity

Time Complexity: O(n)

This means the server work grows linearly with the number of requests it handles.

Common Mistake

[X] Wrong: "The server handles all requests instantly no matter how many come in."

[OK] Correct: Each request takes some time to process, so more requests mean more total work and longer total time.

Interview Connect

Understanding how a web server handles requests helps you explain real-world system behavior clearly and confidently.

Self-Check

What if the server handled multiple requests at the same time using threads? How would the time complexity change?

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 '/'