Button class with callbacks in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when a button press triggers the callback?

Consider a Button class that calls a callback function when pressed. What will be printed when the button is pressed?

Raspberry Pi

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

def greet():
    print("Button pressed!")

btn = Button(greet)
btn.press()

A No output

B Error: callback not defined

C Button pressed!

D Button pressed! Button pressed!

💡 Hint

Think about what the press method does with the callback.

The press method calls the callback function greet, which prints "Button pressed!" once.

🧠 Conceptual

intermediate

Which statement about callbacks in a Button class is true?

In a Button class that accepts a callback function, which of the following is true about how callbacks work?

A Callbacks can only be functions defined inside the Button class.

B The callback function runs immediately when the Button object is created.

C The callback function must return a value to work properly.

D The callback function is stored and called when the button is pressed.

💡 Hint

Think about when the callback is executed.

The callback is saved during initialization and only called when the button is pressed, not before.

🔧 Debug

advanced

Why does this Button callback code raise an error?

What error does the following code raise when btn.press() is called?

Raspberry Pi

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

def greet():
    print("Hello")

btn = Button(greet)
btn.press()

A TypeError: greet() takes 0 positional arguments but 1 was given

B NameError: name 'greet' is not defined

C AttributeError: 'Button' object has no attribute 'callback'

D No error, prints 'Hello'

💡 Hint

Check how the callback is called and how it is defined.

The press method calls self.callback(self), passing one argument, but greet expects none, causing a TypeError.

📝 Syntax

advanced

Which option correctly defines a Button class with a callback that accepts an argument?

Choose the correct Button class definition where the callback function accepts the Button instance as an argument.

A

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

B

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

C

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

D

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

💡 Hint

The callback should be called with exactly one argument: the Button instance.

Option B correctly calls the callback with self once. Others either call with no arguments or wrong arguments.

🚀 Application

expert

What is the output of this Button class with multiple callbacks?

Given the code below, what is printed when btn.press() is called?

Raspberry Pi

class Button:
    def __init__(self):
        self.callbacks = []
    def add_callback(self, cb):
        self.callbacks.append(cb)
    def press(self):
        for cb in self.callbacks:
            cb(self)

def cb1(btn):
    print("First callback")

def cb2(btn):
    print("Second callback")

btn = Button()
btn.add_callback(cb1)
btn.add_callback(cb2)
btn.press()

A First callback\nSecond callback

B First callback

C Second callback\nFirst callback

D No output

💡 Hint

Callbacks are called in the order they were added.

The press method calls each callback in the list in order, so cb1 prints first, then cb2.