Button class with callbacks in Raspberry Pi - Step-by-Step Execution

Concept Flow - Button class with callbacks

Create Button object

↓

Assign callback function

↓

Wait for button press event

↓

Detect button press

↓

Call callback function

↓

Callback runs user code

↓

Wait for next event or exit

This flow shows how a Button object is created, a callback function is assigned, and when the button is pressed, the callback runs automatically.

Execution Sample

Raspberry Pi

import gpiozero

def on_press():
    print('Button pressed!')

button = gpiozero.Button(2)
button.when_pressed = on_press

This code creates a button on GPIO pin 2 and prints a message when the button is pressed.

Execution Table

Step	Action	Evaluation	Result
1	Import gpiozero	Module loaded	gpiozero available
2	Define on_press function	Function created	on_press ready
3	Create Button on pin 2	Button object created	button assigned
4	Assign on_press to when_pressed	Callback linked	button.when_pressed = on_press
5	Wait for button press	Idle	Waiting...
6	Button pressed detected	Event triggered	Callback called
7	Run on_press function	Print executes	Output: 'Button pressed!'
8	Return to wait	Idle	Waiting for next press

💡 Program runs indefinitely until stopped; callbacks run on each button press.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 7	Final
button	None	Button object on pin 2	Callback assigned	Callback executed	Callback ready for next press
on_press	None	Function defined	Function linked	Function ran	Function ready

Key Moments - 2 Insights

Why doesn't the on_press function run immediately when assigned?

Because in the execution_table at Step 4, the function is assigned as a callback, not called. It runs only when the button press event happens at Step 6.

What happens if the button is never pressed?

As shown in Step 5, the program waits idle. The callback never runs until a press event triggers it.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of 'button' after Step 3?

A Button pressed detected

B Button object created on pin 2

C Callback function assigned

D Function on_press executed

💡 Hint

Check the 'Result' column for Step 3 in execution_table.

At which step does the callback function actually run?

A Step 7

B Step 4

C Step 6

D Step 5

💡 Hint

Look for when 'Run on_press function' happens in execution_table.

If the button is never pressed, which step will the program stay at indefinitely?

A Step 7

B Step 6

C Step 5

D Step 4

💡 Hint

See where the program is 'Idle' waiting for input in execution_table.

Concept Snapshot

Button class with callbacks:
- Create Button(pin_number)
- Define callback function
- Assign callback to button.when_pressed
- Callback runs automatically on press
- Program waits for events
- Useful for responsive hardware input

Full Transcript

This example shows how to use a Button class with callbacks on a Raspberry Pi. First, the gpiozero module is imported. Then, a function named on_press is defined to print a message. A Button object is created on GPIO pin 2. The on_press function is assigned as the callback to run when the button is pressed. The program waits for the button press event. When the button is pressed, the callback runs and prints 'Button pressed!'. The program then waits for the next press. This setup allows the program to respond immediately to button presses without constantly checking the button state.

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