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.

Practice

1. What is the main purpose of using the pyzbar library on a Raspberry Pi when working with QR codes?

easy

A. To capture images from the camera

B. To decode and read data from QR codes

C. To display images on the screen

D. To create QR codes from text

Solution

1. Step 1: Understand the role of pyzbar

   The pyzbar library is designed to decode barcodes and QR codes from images.

2. Step 2: Identify its use in QR code reading

   On Raspberry Pi, pyzbar reads the data encoded in QR codes, not capturing or displaying images.

3. Final Answer:

   To decode and read data from QR codes -> Option B

4. Quick Check:

   pyzbar = decode QR codes [OK]

Hint: pyzbar reads QR code data, not captures or displays images [OK]

Common Mistakes:

• Confusing pyzbar with camera capture libraries
• Thinking pyzbar creates QR codes
• Assuming pyzbar displays images

2. Which of the following is the correct way to import the decode function from the pyzbar.pyzbar module in Python on Raspberry Pi?

easy

A. from pyzbar.pyzbar import decode

B. import pyzbar.decode

C. from pyzbar import decode

D. import decode from pyzbar.pyzbar

Solution

1. Step 1: Recall correct Python import syntax

   To import a function from a submodule, use from module.submodule import function.

2. Step 2: Apply to pyzbar

   The decode function is inside pyzbar.pyzbar, so the correct import is from pyzbar.pyzbar import decode.

3. Final Answer:

   from pyzbar.pyzbar import decode -> Option A

4. Quick Check:

   Correct import syntax = from pyzbar.pyzbar import decode [OK]

Hint: Use 'from module.submodule import function' for nested imports [OK]

Common Mistakes:

• Using 'import pyzbar.decode' which is invalid
• Writing 'import decode from ...' which is wrong syntax
• Omitting the submodule pyzbar in import

3. Given the following Python code snippet on Raspberry Pi, what will be printed if the image contains a QR code with data 'Hello123'?

from pyzbar.pyzbar import decode
import cv2

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

medium

A. b'Hello123'

B. obj.data

C. Error: decode() expects a file path

D. Hello123

Solution

1. Step 1: Understand decode output

   The decode function returns a list of objects with a data attribute as bytes.

2. Step 2: Decode bytes to string

   The code calls obj.data.decode('utf-8') to convert bytes to a readable string, so it prints 'Hello123'.

3. Final Answer:

   Hello123 -> Option D

4. Quick Check:

   Decoded bytes to string = Hello123 [OK]

Hint: Decode bytes with .decode('utf-8') to get readable text [OK]

Common Mistakes:

• Printing raw bytes without decoding
• Assuming decode() takes file path instead of image
• Confusing obj.data with string directly

4. What is the error in the following Raspberry Pi Python code for reading a QR code?

from pyzbar.pyzbar import decode
import cv2

image = cv2.imread('qrcode.png')
result = decode('qrcode.png')
print(result)

medium

A. print() cannot print decode results

B. cv2.imread() is missing

C. decode() should receive an image, not a file path string

D. Missing import for numpy

Solution

1. Step 1: Check decode() argument type

   The decode function expects an image object, not a file path string.

2. Step 2: Identify the mistake in code

   The code passes the string 'qrcode.png' directly to decode, which causes an error.

3. Final Answer:

   decode() should receive an image, not a file path string -> Option C

4. Quick Check:

   decode() needs image object, not filename [OK]

Hint: Pass image object to decode(), not filename string [OK]

Common Mistakes:

• Passing filename string directly to decode()
• Forgetting to read image with cv2.imread()
• Assuming decode() reads files itself

5. You want to build a Raspberry Pi program that continuously reads QR codes from a live camera feed and prints the decoded data only once per unique QR code detected. Which approach best solves this problem?

hard

A. Use a set to store decoded QR data and check before printing each frame

B. Print all decoded QR data every frame without filtering duplicates

C. Clear the camera feed buffer after each frame to avoid repeats

D. Restart the camera after each QR code is detected

Solution

1. Step 1: Understand the problem of repeated QR codes

   Live camera frames may detect the same QR code multiple times, causing repeated prints.

2. Step 2: Use a set to track unique QR codes

   Storing decoded data in a set allows checking if a QR code was already printed, avoiding duplicates.

3. Step 3: Avoid inefficient or incorrect methods

   Clearing buffers or restarting camera won't prevent repeated prints logically.

4. Final Answer:

   Use a set to store decoded QR data and check before printing each frame -> Option A

5. Quick Check:

   Use set to track unique QR codes [OK]

Hint: Track seen QR codes in a set to print each once [OK]

Common Mistakes:

• Printing every detection without filtering duplicates
• Trying to clear camera buffer to fix repeats
• Restarting camera unnecessarily