Why does this GPIO button click run twice if I remove the print statement?

Question

I run this script when the Raspberry Pi boots. When a key is pressed (GPIO 19) the interrupt function gets called.

If I remove the print("BUTTON PRESSED") statement on line 10, the interrupt function seems to be called twice in a row for a single button press.

Any idea why this is the case?

import RPi.GPIO as GPIO
import time
import subprocess

GPIO.setmode(GPIO.BCM)

GPIO.setup(19, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def interrupt(channel):
    print("BUTTON PRESSED")
    p = subprocess.Popen(['node', '/btn.js', str(channel)], stdout=subprocess.PIPE)
    out = p.stdout.read()
    print(out)

GPIO.add_event_detect(19, GPIO.FALLING, callback=interrupt, bouncetime=1000)

try:
    while True:
        time.sleep(1)
except:  
    print("Exit GPIO Listener..")
finally
    GPIO.cleanup()

Answer 1

Probably switch bounce if it is a mechanical switch.

What you think of a single event may actually be multiple contacts and releases in a fraction of a second.

Answer 2

All mechanical switches bounce - some e.g. reed switches are more prone to this. There are mechanical solutions used for high current power switches where arcing is an issue but these are inappropriate for logic circuits.

It is possible to minimise bounce - either by suppressing extraneous signals using a capacitor to limit rise time or by introducing delays in software but using changeover switch and bistable circuitry bounce can be absolutely prevented.