The_Official_Raspberry_Pi_-_Beginner’s_Book_Vol1,_2018 (1)

206 THE OFFICIAL RASPBERRY PI BEGINNER'S GUIDE

There are a couple of clever tricks in these three lines of code. The first is in the capture

file name: using % 03 d tells Python to take a number and add as many zeroes to the front as it

needs to make it three digits long. Thus ‘1’ becomes ‘001’, ‘2’ becomes ‘002’, and ‘10’ becomes

‘010’. You need this in your program to keep your files in the correct order and to make sure

you’re not writing over a file you’ve already saved.

The % frame at the end of that line tells Python to use the number of the frame variable in

the file name. To make sure each file is unique, the last line – frame += 1 – increments the

frame variable by 1. The first time you press the button, frame will be increased from 1 to 2;

the next time, from 2 to 3; and so on.

At the moment, though, your code won’t exit cleanly when you’re finished taking pictures.

To make that happen, you need an except for your try. Type the following, remembering to

remove one level of indentation on the first line so Python knows it’s not part of the try section:

except KeyboardInterrupt:

camera.stop_preview()

break

Your finished program will look like this:

from picamera import PiCamera

from time import sleep

from gpiozero import Button

camera = PiCamera()

button = Button( 2 )

camera.start_preview()

frame = 1

while True:

try:

button.wait_for_press()

camera.capture('/home/pi/Desktop/animation/frame%03d.jpg'

% frame)

frame += 1

except KeyboardInterrupt:

camera.stop_preview()

break

Try clicking Run, but instead of pressing the button, press the CTRL and C keys on the

keyboard. You don’t need to press both keys at the same time: just hold down CTRL, press

and release C, then release CTRL. These two keys act as an interrupt, telling Python to

stop what it’s doing. Without the except KeyboardInterrupt: line, Python would

immediately quit and leave the camera preview blocking your screen; with that line in place,