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I am making a Raspberry Pi librarian which can keep track of available books and who has taken out a book. Users scan NFC cards to begin the process of borrowing a book.

The main program is an infinite loop and goes like this:

while True:
    # wait for card to be scanned
    # identify the user from a database
    # ask if borrowing or returning
    # ask which book user wants to borrow/return
    # update the database

I want to be able to include a physical cancel button so that the user can cancel at any time in the borrowing/returning process and the main loop will restart and wait for a card again.

I've used this guide from Raspi.TV and successfully set up an interrupt button.

How would I code this into my program?

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You need to check the state of the GPIO at regular points in your main loop and if necessary issue the continue command to restart the loop.

e.g.

if read(MY_INTERRUPT_GPIO) == 1: # high means restart
   continue

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