QR code reading in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of QR code reading snippet

What will be the output of this Python code snippet that reads a QR code image using OpenCV and pyzbar on a Raspberry Pi?

Raspberry Pi

import cv2
from pyzbar.pyzbar import decode

img = cv2.imread('qrcode.png')
detected = decode(img)
for obj in detected:
    print(obj.data.decode('utf-8'))

A Raises FileNotFoundError because 'qrcode.png' does not exist

B Prints nothing because decode returns an empty list

C Prints the raw bytes of the QR code without decoding

D Prints the decoded text from the QR code in 'qrcode.png'

💡 Hint

Check what decode() returns and how obj.data is processed.

The decode function returns a list of detected QR code objects. Each object's data attribute contains the QR code content in bytes, which is decoded to a string with utf-8.

🧠 Conceptual

intermediate

Understanding QR code detection libraries

Which library is commonly used on Raspberry Pi to decode QR codes from images in Python?

A matplotlib

B pyzbar

C numpy

D pygame

💡 Hint

It is a wrapper around the ZBar library.

pyzbar is a Python wrapper for the ZBar library, which can decode barcodes and QR codes from images.

🔧 Debug

advanced

Identify the error in QR code reading code

What error will this code produce when run on a Raspberry Pi with OpenCV and pyzbar installed? import cv2 from pyzbar.pyzbar import decode img = cv2.imread('qrcode.png') result = decode(img) print(result.data.decode('utf-8'))

A AttributeError: 'list' object has no attribute 'data'

B FileNotFoundError: 'qrcode.png' not found

C TypeError: decode() missing required positional argument

D No error, prints decoded QR code text

💡 Hint

Check the type of the object returned by decode().

decode() returns a list of detected QR code objects. Accessing .data directly on the list causes AttributeError.

📝 Syntax

advanced

Correct syntax for reading QR code from camera

Which code snippet correctly reads a frame from the Raspberry Pi camera using OpenCV and decodes a QR code?

A

import cv2
from pyzbar.pyzbar import decode
cap = cv2.VideoCapture(0)
ret, frame = cap.read
result = decode(frame)
print(result.data.decode('utf-8'))
cap.release()

B

import cv2
from pyzbar.pyzbar import decode
cap = cv2.VideoCapture(0)
frame = cap.read()
detected = decode(frame)
for d in detected:
    print(d.data.decode('utf-8'))
cap.release()

C

import cv2
from pyzbar.pyzbar import decode
cap = cv2.VideoCapture(0)
ret, frame = cap.read()
detected = decode(frame)
for d in detected:
    print(d.data.decode('utf-8'))
cap.release()

D

import cv2
from pyzbar.pyzbar import decode
cap = cv2.VideoCapture(0)
ret, frame = cap.read()
result = decode(frame)
print(result.data.decode('utf-8'))
cap.release()

💡 Hint

Check the return value of cap.read() and how to handle decode results.

cap.read() returns a tuple (ret, frame). decode returns a list, so you must iterate to access .data. Option C correctly does both.

🚀 Application

expert

Number of QR codes detected in an image

Given this code snippet running on Raspberry Pi, how many QR codes will be detected and printed if the image contains three distinct QR codes? import cv2 from pyzbar.pyzbar import decode img = cv2.imread('multi_qr.png') detected = decode(img) print(len(detected))

A 3

B 1

C 0

D Raises an error

💡 Hint

decode() returns a list of all detected QR codes in the image.

The decode function detects all QR codes in the image and returns them as a list. The length of this list equals the number of QR codes detected.