How to add a delay between the button press, and PiCamera taking a photo, using sleep().

picamera_sleep_demo.py

from time import sleep
import RPi.GPIO as GPIO
import picamera

#... etc.
# Refer related gists, for how to setup the camera ...etc.

#Wait for someone to push the button
while True:

    #Check to see if button is pushed
    is_pressed = GPIO.wait_for_edge(21, GPIO.FALLING, timeout=100) #Check if Pin 21 is pressed

    #Stay inside loop until button is pressed
    if is_pressed is None:
        continue # Go back to check if the button is pressed again.

    #Button has been pressed!
    print("Button pressed!")
    camera.start_preview(resolution=(screen_w, screen_h)) #Start camera preview

    #Wait for 2 seconds.
    sleep(2)

    #Save photo
    camera.capture('my_file.jpg')

    #restart loop
    camera.stop_preview()
    is_pressed = False