← QCE Digital Solutions Unit 1

Responding to keyboard events

Links a key press to a handler function. Each time a key is pressed, tkinter calls handler(event), and event.keysym tells you which key it was.

Part of: QCE Digital Solutions Unit 1 — Algorithms and Programming Techniques — Module 92

Understanding Responding to keyboard events in QCE Digital Solutions Unit 1

What it does: Tkinter calls handler(event), so the handler needs one parameter (here event) to receive the key press details. Comparing event.keysym with a key name inside the handler lets the program react to one particular key. Pass the function name without brackets. Brackets would call on_key straight away instead of letting tkinter call it when a key is pressed. For the arrow keys, keysym is Up, Down, Left or Right. Use == to compare values. A single = is assignment and causes a syntax error in a condition. The handler is an event handler. It waits quietly and runs every time the event happens.

Common mistakes: A function header must end with a colon. The first line should be def key(event): so the handler body can follow. Python is case sensitive. The window class is Tk with a capital T, so it must be root = tk.Tk(). Without it, root.bind() has no window to work on.

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Learning

Duo

Which line makes tkinter call on_key every time any key is pressed?

root.bind('<KeyPress>', on_key)root.bind('<KeyPress>', on_key())

Matches

root.bind('<KeyPress>', on_key())

↓

The handler is called with brackets, so it runs straight away instead of waiting for a key press. Pass on_key without ().

Applying

Meanings

Term

root.bind('<KeyPress>', handler)

Meaning

Links a key press to a handler function. Each time a key is pressed, tkinter calls handler(event), and event.keysym tells you which key it was.

Practice

import tkinter as tk

Testing

Gaps

root.('<KeyPress>', handler)

Check Answer

Spots

1def key(event)

2 print(event.keysym)

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Quick Questions

1. Which line makes tkinter call on_key every time any key is pressed?

A. root.bind('<KeyPress>', on_key) ✓

B. root.bind('<KeyPress>', on_key())

Pass the function name without brackets. Brackets would call on_key straight away instead of letting tkinter call it when a key is pressed.

2. A handler for a key press needs which parameter?

A. def on_key(event): ✓

B. def on_key():

tkinter passes an event object to the handler, so the function must have a parameter to receive it.

3. Which expression tells you the name of the key that was pressed?

A. root.keysym

B. event.keysym ✓

The key name is stored on the event object that tkinter passes to the handler, not on the window.

4. A player presses the Up arrow key. What is event.keysym?

A. 'Arrow'

B. 'Up' ✓

For the arrow keys, keysym is Up, Down, Left or Right.

5. Which condition checks that the Escape key was pressed inside the handler?

A. if event.keysym == 'Escape': ✓

B. if event.keysym = 'Escape':

Use == to compare values. A single = is assignment and causes a syntax error in a condition.

Match the Pairs

root.bind('<KeyPress>', on_key) → Tells tkinter to call the function on_key every time any key is pressed in the window

event.keysym → Gives the name of the key that was pressed, such as 'a', 'Up' or 'space'

A handler function must have a parameter to receive the event that tkinter passes in → def on_key(event):

Respond only when a specific key, such as the Up arrow, is pressed → if event.keysym == 'Up':

root.bind('<KeyPress>', on_key()) → The handler is called with brackets, so it runs straight away instead of waiting for a key press. Pass on_key without ().

def on_key(): print(event.keysym) → The handler has no event parameter, so event is undefined and tkinter cannot pass the key press in. It should be def on_key(event):

Key Term

root.bind('<KeyPress>', handler)

Links a key press to a handler function. Each time a key is pressed, tkinter calls handler(event), and event.keysym tells you which key it was.

Fill in the Blanks

1. root.bind('<KeyPress>', handler)

Hint: Type the method that links an event to a handler function

2. def handler(event): print(event.keysym)

Hint: Type the parameter name that receives the details of the key press

3. key = event.keysym

Hint: Type the attribute that holds the name of the key pressed

4. if event.keysym == 'Up': print('Up arrow')

Hint: Type the operator that checks whether the key name equals a value

5. def press(event): print(event.keysym) root.bind('<KeyPress>', press)

Hint: Type the name of the function defined above, without brackets

Spot the Error

Question 1

A def key(event)

B print(event.keysym)

C root.bind('<KeyPress>', key)

Line A — A function header must end with a colon. The first line should be def key(event): so the handler body can follow.

Question 2

A import tkinter as tk

B root = tk.tk()

C def key(event):

D print(event.keysym)

E root.bind('<KeyPress>', key)

Line B — Python is case sensitive. The window class is Tk with a capital T, so it must be root = tk.Tk(). Without it, root.bind() has no window to work on.

Question 3

A root = tk.Tk()

B def key(event):

C print(event.keysym)

D root.bind('<KeyPress>', key())

E root.mainloop()

Line D — bind() needs the handler function itself, not the result of calling it. Writing key() runs it straight away. It should be root.bind('<KeyPress>', key).

Question 4

A def key(event):

B if event.keysym == 'Up':

C print('Moving up'

D root.bind('<KeyPress>', key)

Line C — The print call is missing its closing bracket, which causes a SyntaxError. It should be print('Moving up').

Question 5

A def key(event):

B pressed = event.keysym

C if pressed = 'space':

D print('Jump!')

E root.bind('<KeyPress>', key)

Line C — A single = assigns a value, but an if condition needs the comparison operator ==. It should be if pressed == 'space':.

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