Serving sensor data as JSON API in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Flask API endpoint?

Consider this Python Flask code running on a Raspberry Pi that reads a temperature sensor and serves the data as JSON. What will be the output when a client sends a GET request to /temperature?

Raspberry Pi

from flask import Flask, jsonify
app = Flask(__name__)

def read_temperature():
    return 23.5

@app.route('/temperature')
def temperature():
    temp = read_temperature()
    return jsonify({'temperature_celsius': temp})

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

A {"temp": 23.5}

B {"temperature_celsius": 23.5}

C 23.5

D Internal Server Error

💡 Hint

Look at the dictionary key used in jsonify.

The Flask endpoint returns a JSON response with key 'temperature_celsius' and value 23.5. So the output JSON string is {"temperature_celsius": 23.5}.

🧠 Conceptual

intermediate

Which HTTP method should be used to request sensor data from a JSON API?

You have a Raspberry Pi serving sensor data as a JSON API. Which HTTP method is the correct and standard way to request this data?

A GET

B DELETE

C PUT

D POST

💡 Hint

Think about which method is used to retrieve data without changing it.

GET is the standard HTTP method used to retrieve data from a server without modifying it.

🔧 Debug

advanced

What error does this Raspberry Pi Flask code raise?

This code tries to serve sensor data as JSON but has a bug. What error will it raise when running?

Raspberry Pi

from flask import Flask, jsonify
app = Flask(__name__)

def read_humidity():
    return 55

@app.route('/humidity')
def humidity():
    value = read_humidity()
    return jsonify({'humidity': value})

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

A SyntaxError

B TypeError

C KeyError

D No error, returns JSON

💡 Hint

Check the syntax of the argument passed to jsonify.

The code uses jsonify('humidity': value) which is invalid syntax. The correct syntax is jsonify({'humidity': value}). This causes a SyntaxError.

🚀 Application

advanced

How to add a timestamp to sensor JSON response?

You want to serve sensor data with a timestamp in the JSON response on your Raspberry Pi Flask API. Which code snippet correctly adds the current time in ISO format under key 'timestamp'?

Raspberry Pi

from flask import Flask, jsonify
from datetime import datetime
app = Flask(__name__)

def read_light():
    return 300

@app.route('/light')
def light():
    value = read_light()
    # Add timestamp here
    return jsonify({})

A return jsonify({'light': value, 'timestamp': str(datetime.now)})

B return jsonify({'light': value, 'timestamp': datetime.isoformat()})

C return jsonify({'light': value, 'timestamp': datetime.now})

D return jsonify({'light': value, 'timestamp': datetime.now().isoformat()})

💡 Hint

Use datetime.now() to get current time, then convert to ISO string.

datetime.now() returns current datetime object. Calling isoformat() on it returns a string in ISO format. This is the correct way to add a timestamp string.

❓ Predict Output

expert

What is the output of this asynchronous sensor data API code?

This Raspberry Pi code uses async Flask to serve sensor data. What is the JSON output when calling /sensor?

Raspberry Pi

import asyncio
from flask import Flask, jsonify
app = Flask(__name__)

async def read_sensor():
    await asyncio.sleep(0.1)
    return {'temperature': 22.1, 'humidity': 48}

@app.route('/sensor')
async def sensor():
    data = await read_sensor()
    return jsonify(data)

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

A {"temperature": "22.1", "humidity": "48"}

B RuntimeError

C {"temperature": 22.1, "humidity": 48}

D Empty JSON {}

💡 Hint

Check the awaited function and returned dictionary keys and values.

The async function returns a dictionary with float and int values. jsonify converts it to JSON with numeric values, so output is {"temperature": 22.1, "humidity": 48}.

Practice

1.

What is the main purpose of serving sensor data as a JSON API on a Raspberry Pi?

easy

A. To allow other devices to easily access sensor information in a standard format

B. To store sensor data only on the Raspberry Pi without sharing

C. To convert sensor data into images for display

D. To run heavy calculations on sensor data locally

Solution

1. Step 1: Understand JSON API purpose

   A JSON API provides data in a simple, standard format that other devices can read easily.

2. Step 2: Relate to sensor data sharing

   Serving sensor data as JSON means sharing sensor readings so other devices or apps can use them.

3. Final Answer:

   To allow other devices to easily access sensor information in a standard format -> Option A

4. Quick Check:

   JSON API shares sensor data easily [OK]

Hint: JSON API means sharing data in a simple format [OK]

Common Mistakes:

• Thinking JSON API stores data only locally
• Confusing JSON API with image or video output
• Assuming JSON API does heavy local processing

2.

Which of the following is the correct way to import Flask in a Python script on Raspberry Pi?

?

easy

A. import flask

B. import Flask

C. from Flask import flask

D. from flask import Flask

Solution

1. Step 1: Recall Flask import syntax

   The Flask class is imported from the flask module using: from flask import Flask

2. Step 2: Check options for correct case and syntax

   from flask import Flask matches correct syntax and case; others have wrong case or structure.

3. Final Answer:

   from flask import Flask -> Option D

4. Quick Check:

   Correct Flask import uses 'from flask import Flask' [OK]

Hint: Flask class is imported with 'from flask import Flask' [OK]

Common Mistakes:

• Using wrong capitalization like 'import Flask'
• Importing the whole module without accessing Flask class
• Mixing module and class names incorrectly

3.

What will be the output of this Flask route when accessed?

from flask import Flask, jsonify
app = Flask(__name__)

@app.route('/sensor')
def sensor_data():
    data = {'temperature': 22.5, 'humidity': 60}
    return jsonify(data)

# Assume app is running

medium

A. Error: jsonify not defined

B. {temperature: 22.5, humidity: 60}

C. {"temperature": 22.5, "humidity": 60}

D. ['temperature', 22.5, 'humidity', 60]

Solution

1. Step 1: Understand jsonify output

   jsonify converts Python dict to JSON string with keys in quotes and proper JSON format.

2. Step 2: Match output format

   {"temperature": 22.5, "humidity": 60} shows correct JSON string with keys and values properly formatted.

3. Final Answer:

   {"temperature": 22.5, "humidity": 60} -> Option C

4. Quick Check:

   jsonify returns JSON string with quoted keys [OK]

Hint: jsonify outputs JSON with quoted keys and values [OK]

Common Mistakes:

• Forgetting quotes around keys in JSON output
• Confusing Python dict print with JSON string
• Assuming jsonify returns Python list

4.

Identify the error in this Flask code snippet that serves sensor data as JSON:

from flask import Flask, jsonify
app = Flask(__name__)

@app.route('/data')
def data():
    sensor = {'temp': 25}
    return json.dumps(sensor)

app.run()

medium

A. json.dumps is used instead of jsonify

B. Flask app is not created correctly

C. Route decorator syntax is wrong

D. Missing import for Flask

Solution

1. Step 1: Check JSON response method

   Flask expects jsonify to create proper response with correct headers, not json.dumps.

2. Step 2: Identify error cause

   Using json.dumps returns a string without correct content-type, causing client issues.

3. Final Answer:

   json.dumps is used instead of jsonify -> Option A

4. Quick Check:

   Use jsonify, not json.dumps, for Flask JSON responses [OK]

Hint: Use jsonify for Flask JSON responses, not json.dumps [OK]

Common Mistakes:

• Returning json.dumps output directly in Flask
• Not importing jsonify
• Assuming json.dumps sets response headers

5.

You want to serve live temperature sensor data from a Raspberry Pi using Flask. Which approach correctly updates the JSON response with real sensor readings each time the API is called?

from flask import Flask, jsonify
app = Flask(__name__)

def read_temp_sensor():
    # Returns current temperature reading
    return 24.7

@app.route('/temp')
def temp():
    # Fill in here

app.run()

hard

A. temp = 24.7
return jsonify({'temperature': temp})

B. temp = read_temp_sensor()
return jsonify({'temperature': temp})

C. return jsonify({'temperature': read_temp_sensor})

D. return {'temperature': read_temp_sensor()} # Flask auto converts

Solution

1. Step 1: Call sensor function inside route

   We must call read_temp_sensor() to get current value each time the route is accessed.

2. Step 2: Return JSON response with current value

   Assign the value to a variable and pass it to jsonify to return proper JSON response.

3. Final Answer:

   temp = read_temp_sensor() return jsonify({'temperature': temp}) -> Option B

4. Quick Check:

   Call sensor function and jsonify result each request [OK]

Hint: Call sensor function inside route, then jsonify result [OK]

Common Mistakes:

• Returning function object instead of calling it
• Using fixed value instead of live sensor reading
• Returning dict without jsonify in Flask