Can someone please explain to me this code?

[Soumen banerjee]

Hello,
I was trying to find a method to make global hotkeys with python in
linux. I found this one which uses a library called python-xlib. The
point is that since i dont have much experience with this, i cant
understand some of the code. Can someone please explain to me how this
code works? According to the author, it is meant to reduce and
increase volume. Im not interested in that specifically. All i want is
to be ale to bind a hotkey to a function in my python program.

from Xlib.display import Display
from Xlib import X
import oss

# custom keys from my dell D400 Laptop
vol_plus = 176
vol_moins = 174

keys = [vol_plus,vol_moins]

def changeVolume(aValue):
mixer = oss.open_mixer()
symbol = oss.SOUND_DEVICE_LABELS.index('Vol ')
left,right = mixer.read_channel(symbol)

avg = (left + right) / 2
if (avg + aValue) >= 0:
mixer.write_channel(symbol,(left + aValue,right + aValue))
mixer.close()

def handle_event(aEvent):
keycode = aEvent.detail
if aEvent.type == X.KeyPress:
if keycode == vol_moins:
changeVolume(-2)
elif keycode == vol_plus:
changeVolume(+2)

def main():
# current display
disp = Display()
root = disp.screen().root

# we tell the X server we want to catch keyPress event
root.change_attributes(event_mask = X.KeyPressMask)

for keycode in keys:
root.grab_key(keycode, X.AnyModifier, 1,X.GrabModeAsync, X.GrabModeAsync)

while 1:
event = root.display.next_event()
handle_event(event)

if __name__ == '__main__':
main()
Regards
Soumen

 

[CTO]

Hello,
I was trying to find a method to make global hotkeys with python
in
linux. I found this one which uses a library called python-xlib.
The
point is that since i dont have much experience with this, i cant
understand some of the code. Can someone please explain to me how
this
code works? According to the author, it is meant to reduce and
increase volume. Im not interested in that specifically. All i
want is
to be ale to bind a hotkey to a function in my python program.


It's not so hard. Just a little confusing at first. The thing is that
X is a
client-server protocol, so it doesn't call a function or anything to
let
you know that the user has pressed a key- it just sends all the events
you've asked for to your application, and lets it handle them how it
will. To get a better idea about how that happens, fire up xev (the
X Event Viewer) on your console.

from Xlib.display import Display
from Xlib import X


You still need these


def handle_event(aEvent):
keycode = aEvent.detail
if aEvent.type == X.KeyPress:
if keycode == vol_moins:
changeVolume(-2)
elif keycode == vol_plus:
changeVolume(+2)


(Spacing mine)
You don't really care about this, but the important thing is how
it's structured. It takes an event, then gets its detail attribute,
which is where the key that is pressed will go if it is a keypress,
then sees if its type is KeyPress, then performs the appropriate
action based on that keycode. Your app will do pretty much the
same thing, just with different actions.


def main():
# current display
disp = Display()
root = disp.screen().root

# we tell the X server we want to catch keyPress event
root.change_attributes(event_mask = X.KeyPressMask)

for keycode in keys:
root.grab_key(keycode, X.AnyModifier, 1,X.GrabModeAsync,
X.GrabModeAsync)

while 1:
event = root.display.next_event()
handle_event(event)

if __name__ == '__main__':
main()


(spacing mine)
While I'd do things a little differently, this is pretty much
boilerplate.
You can use this as a template for what you want to do, assuming
that you've appropriately defined your keycodes and handle_event
function.

Geremy Condra

 

[Soumen banerjee]

Hello,
I'd like a little more help with the following lines:

root.change_attributes(event_mask = X.KeyPressMask)

for keycode in keys:
root.grab_key(keycode, X.AnyModifier, 1,X.GrabModeAsync,
X.GrabModeAsync)

what exactly do they do?
regards
Soumen

 

[CTO]

root.change_attributes(event_mask = X.KeyPressMask)

This asks X to send this application keypress events

root.grab_key(keycode, X.AnyModifier, 1,X.GrabModeAsync, X.GrabModeAsync)

This tells X to grab the keyboard if the given keycode is generated
and
any modifier is pressed, not to stop processing keyboard events, and
not
to stop processing pointer events. That's pretty safe, although I'm
not
sure if you'd need the modifier for your application.