Is a Python event polled or interrupt driven?

[Chris Angelico]

On Fri, 13 Oct 2023 at 01:48, Chris Green via Python-list
<python-list at python.org> wrote:
>
> In the following code is the event polled by the Python process
> running the code or is there something cleverer going on such that
> Python sees an interrupt when the input goes high (or low)?
>

This isn't something inherent to Python; it's the specific behaviour
of the library you're using. So I dug through that library a bit, and
ended up here:

https://github.com/adafruit/adafruit-beaglebone-io-python/blob/cf306ed7f9f24111d0949dd60ac232e81241bffe/source/event_gpio.c#L753

which starts a thread:

https://github.com/adafruit/adafruit-beaglebone-io-python/blob/cf306ed7f9f24111d0949dd60ac232e81241bffe/source/event_gpio.c#L662

which appears to make use of epoll for efficient event handling. Edge
detection itself seems to be done here:

https://github.com/adafruit/adafruit-beaglebone-io-python/blob/cf306ed7f9f24111d0949dd60ac232e81241bffe/source/event_gpio.c#L522

I don't know enough about the architecture of the BeagleBone to be
certain, but my reading of it is that most of the work of edge
detection is done by the OS kernel, which then sends the Adafruit
handler a notification via a file descriptor. The secondary thread
waits for those messages (which can be done very efficiently), and in
turn calls the Python callbacks.

In other words, the "something cleverer" is all inside the OS kernel,
and yes, in effect, it's an interrupt.

ChrisA