Button class with callbacks in Raspberry Pi - Mini Project: Build & Apply

Button class with callbacks

📖 Scenario: You are building a simple program to handle button presses on a Raspberry Pi. Buttons can do different things when pressed, so you want to create a Button class that can run a special function (called a callback) whenever the button is pressed.This is like having a doorbell that plays a different sound depending on who is at the door. The button press triggers the sound.

🎯 Goal: Create a Button class that stores a callback function. When the button is pressed, it should run the callback. You will then create a button instance, assign a callback, and simulate a button press to see the callback run.

📋 What You'll Learn

Create a Button class with an __init__ method that takes a callback parameter and stores it.

Add a press method to the Button class that calls the stored callback function.

Create a function called say_hello that prints 'Hello! Button pressed.'.

Create a Button instance called button with say_hello as the callback.

Call the press method on the button instance to run the callback.

Print the output of the callback when the button is pressed.

💡 Why This Matters

🌍 Real World

Buttons on Raspberry Pi or other devices often need to run specific code when pressed. Using callbacks lets you easily change what happens without rewriting the button code.

💼 Career

Understanding callbacks and event-driven programming is important for embedded systems, hardware interfacing, and GUI programming jobs.

1

Create the Button class with callback storage

Create a class called Button with an __init__ method that takes a parameter called callback and stores it in self.callback.

Raspberry Pi

# Create the Button class with __init__ method
# Your code here

Hint

Remember, __init__ is the method that runs when you create a new object. Store the callback function in self.callback.

2

Add the press method to call the callback

Add a method called press to the Button class that calls the stored callback function using self.callback().

Raspberry Pi

class Button:
    def __init__(self, callback):
        self.callback = callback

    # Add the press method below
    # Your code here

Hint

The press method should run the callback function stored in self.callback. Just call it like a normal function.

3

Create a callback function say_hello

Create a function called say_hello that prints 'Hello! Button pressed.' when called.

Raspberry Pi

class Button:
    def __init__(self, callback):
        self.callback = callback

    def press(self):
        self.callback()

# Create the say_hello function below
# Your code here

Hint

Define a simple function that prints the message exactly as shown.

4

Create Button instance and press it to run callback

Create a Button instance called button with say_hello as the callback. Then call button.press() to run the callback and print the message.

Raspberry Pi

class Button:
    def __init__(self, callback):
        self.callback = callback

    def press(self):
        self.callback()

def say_hello():
    print('Hello! Button pressed.')

# Create button instance and press it below
# Your code here

Hint

Create the button with say_hello as callback and call press() to see the message.

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