Button class with callbacks in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the GPIO library for Raspberry Pi.

Raspberry Pi

import [1] as GPIO

Drag options to blanks, or click blank then click option'

Asys

Bgpiozero

Ctime

DRPi.GPIO

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'gpiozero' instead of 'RPi.GPIO' when the task expects the GPIO library import.

Importing unrelated libraries like 'time' or 'sys'.

The RPi.GPIO library is used to control GPIO pins on Raspberry Pi.

2fill in blank

medium

Complete the code to set the GPIO mode to BCM numbering.

Raspberry Pi

GPIO.setmode([1])

Drag options to blanks, or click blank then click option'

AGPIO.BCM

BGPIO.OUT

CGPIO.BOARD

DGPIO.IN

Attempts:

3 left

💡 Hint

Common Mistakes

Using GPIO.BOARD instead of GPIO.BCM.

Using GPIO.OUT or GPIO.IN which are for pin direction, not mode.

Setting GPIO mode to GPIO.BCM uses the Broadcom chip pin numbers.

3fill in blank

hard

Fix the error in setting up a GPIO pin 17 as input with a pull-up resistor.

Raspberry Pi

GPIO.setup(17, [1], pull_up_down=GPIO.PUD_UP)

Drag options to blanks, or click blank then click option'

AGPIO.OUT

BGPIO.IN

CGPIO.PUD_DOWN

DGPIO.PUD_UP

Attempts:

3 left

💡 Hint

Common Mistakes

Setting pin as GPIO.OUT when using pull-up resistor.

Confusing pull_up_down parameter with pin direction.

Pin 17 should be set as input (GPIO.IN) to use a pull-up resistor.

4fill in blank

hard

Fill both blanks to add an event detection on pin 18 for falling edge and assign a callback function.

Raspberry Pi

GPIO.add_event_detect(18, [1], callback=[2])

Drag options to blanks, or click blank then click option'

AGPIO.FALLING

BGPIO.RISING

Cbutton_pressed

Dbutton_released

Attempts:

3 left

💡 Hint

Common Mistakes

Using GPIO.RISING instead of GPIO.FALLING for falling edge detection.

Using wrong callback function name.

The event detection is set for falling edge (GPIO.FALLING) and the callback function is button_pressed.

5fill in blank

hard

Fill all three blanks to define a Button class with an __init__ method that sets pin, callback, and configures GPIO input with pull-up.

Raspberry Pi

class Button:
    def __init__(self, pin, callback):
        self.pin = [1]
        self.callback = [2]
        GPIO.setup(self.pin, GPIO.IN, pull_up_down=[3])

Drag options to blanks, or click blank then click option'

Apin

Bcallback

CGPIO.PUD_UP

DGPIO.PUD_DOWN

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning wrong variables to self.pin or self.callback.

Using pull-down resistor instead of pull-up.

The __init__ method assigns the pin and callback parameters to instance variables and sets up the pin as input with pull-up resistor.

Practice

1.

What is the main purpose of using a Button class with callbacks in Raspberry Pi programming?

easy

A. To run specific functions automatically when the button is pressed or released

B. To make the button physically larger

C. To change the button's color on the screen

D. To connect the button to the internet

Solution

1. Step 1: Understand what callbacks do

   Callbacks are functions that run automatically when an event happens, like pressing a button.

2. Step 2: Connect callbacks to button actions

   The Button class uses callbacks to run code when the button is pressed or released, making the program respond to these events.

3. Final Answer:

   To run specific functions automatically when the button is pressed or released -> Option A

4. Quick Check:

   Callbacks run functions on events = A [OK]

Hint: Callbacks run code on button press/release events [OK]

Common Mistakes:

• Thinking callbacks change button size or color
• Confusing callbacks with hardware features
• Assuming callbacks connect buttons to the internet

2.

Which of the following is the correct way to assign a function my_function to run when a button is pressed using a Button class?

button = Button(17)
# What goes here?

easy

A. button.on_press(my_function)

B. button.on_press = my_function()

C. button.on_press = my_function

D. button.on_press: my_function

Solution

1. Step 1: Understand function assignment

   Assigning a function as a callback means giving the function name without parentheses, so it can be called later.

2. Step 2: Check syntax for callback assignment

   Using button.on_press = my_function assigns the function correctly. Adding parentheses calls it immediately, which is wrong.

3. Final Answer:

   button.on_press = my_function -> Option C

4. Quick Check:

   Assign function without () for callback = A [OK]

Hint: Assign function name without () for callbacks [OK]

Common Mistakes:

• Adding parentheses calls function immediately
• Using parentheses like a function call in assignment
• Using incorrect syntax like colons or parentheses

3.

What will be the output when the following code runs and the button is pressed once?

class Button:
    def __init__(self):
        self.on_press = None
    def press(self):
        if self.on_press:
            self.on_press()

def greet():
    print('Hello!')

button = Button()
button.on_press = greet
button.press()

medium

A. Hello!

B. None

C. Error: on_press is not callable

D. No output

Solution

1. Step 1: Understand the press method

   The press method checks if on_press is set and calls it if so.

2. Step 2: Trace the code execution

   The greet function prints 'Hello!'. It is assigned to on_press, so pressing the button calls greet and prints 'Hello!'.

3. Final Answer:

   Hello! -> Option A

4. Quick Check:

   Callback runs greet() printing Hello! = C [OK]

Hint: Callbacks run assigned function on press, printing output [OK]

Common Mistakes:

• Forgetting to assign the function before pressing
• Expecting no output if on_press is None
• Confusing function call with function assignment

4.

Find the error in this code snippet that uses a Button class with callbacks:

class Button:
    def __init__(self):
        self.on_press = None
    def press(self):
        self.on_press()

button = Button()
button.press()

medium

A. Button class is missing the on_release callback

B. on_press is called without checking if it is None, causing an error

C. The press method should not call on_press at all

D. The button object is not created correctly

Solution

1. Step 1: Analyze the press method

   The press method calls self.on_press() without checking if on_press is set.

2. Step 2: Identify the error cause

   Since on_press is None by default and never assigned, calling None() causes a TypeError.

3. Final Answer:

   on_press is called without checking if it is None, causing an error -> Option B

4. Quick Check:

   Calling None as function causes error = B [OK]

Hint: Always check callback is set before calling it [OK]

Common Mistakes:

• Ignoring None check before calling callback
• Assuming on_press is set automatically
• Thinking missing on_release causes this error

5.

You want to create a Button class that supports both on_press and on_release callbacks. Which code correctly implements this so pressing and releasing the button calls the right functions?

class Button:
    def __init__(self):
        # What should go here?
    def press(self):
        # What should go here?
    def release(self):
        # What should go here?

button = Button()
button.on_press = lambda: print('Pressed')
button.on_release = lambda: print('Released')

button.press()
button.release()

hard

A. Initialize on_press and on_release as True; check if True before calling

B. Initialize on_press and on_release to empty strings; print them in press and release

C. Do not initialize callbacks; call them directly in press and release methods

D. Initialize on_press and on_release to None; call them if set in press and release methods

Solution

1. Step 1: Initialize callbacks properly

   Set self.on_press and self.on_release to None in __init__ to indicate no function assigned yet.

2. Step 2: Call callbacks safely in methods

   In press and release, check if the callback is not None before calling it to avoid errors.

3. Final Answer:

   Initialize on_press and on_release to None; call them if set in press and release methods -> Option D

4. Quick Check:

   Initialize None and check before call = D [OK]

Hint: Set callbacks to None and check before calling [OK]

Common Mistakes:

• Using wrong default values like empty strings or True
• Calling callbacks without checking if assigned
• Not supporting both press and release callbacks