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.