`evdev` Not Correctly Capturing LEFTSHIFT Key Presses

[mroavi]

I've encountered an issue with evdev in my application, which stopped working as expected. The problem is that evdev is not capturing LEFTSHIFT key presses correctly. Adding a small delay after the press and release of the LEFTSHIFT key resolves the issue. Below is a minimal example that reproduces the problem:

# This code does not work as expected. It emits a lowercase 'a', whereas it should emit an uppercase 'A'.
from evdev import UInput, InputDevice, ecodes as e
import time

keybd = '/dev/input/event18'

ui = UInput.from_device(keybd, name='keyboard-device')
ui.capabilities(verbose=True).keys()

ui.write(e.EV_KEY, e.KEY_LEFTSHIFT, 1)  # KEY_LEFTSHIFT down
ui.syn()
ui.write(e.EV_KEY, e.KEY_A, 1)   # KEY_A down
ui.write(e.EV_KEY, e.KEY_A, 0)   # KEY_A up
ui.syn()
ui.write(e.EV_KEY, e.KEY_LEFTSHIFT, 0)  # KEY_LEFTSHIFT up
ui.syn()

ui.close()

Adding small delays after the LEFTSHIFT press and release fixes the issue:

# This code works as expected. It emits an uppercase 'A'.
from evdev import UInput, InputDevice, ecodes as e
import time

keybd = '/dev/input/event18'

ui = UInput.from_device(keybd, name='keyboard-device')
ui.capabilities(verbose=True).keys()

ui.write(e.EV_KEY, e.KEY_LEFTSHIFT, 1)  # KEY_LEFTSHIFT down
time.sleep(0.05)  # <-- FIXES THE PROBLEM
ui.syn()
ui.write(e.EV_KEY, e.KEY_A, 1)  # KEY_A down
ui.write(e.EV_KEY, e.KEY_A, 0)  # KEY_A up
ui.syn()
ui.write(e.EV_KEY, e.KEY_LEFTSHIFT, 0)  # KEY_LEFTSHIFT up
time.sleep(0.05)  # <-- FIXES THE PROBLEM
ui.syn()

ui.close()

This behavior suggests that evdev may require a brief period to correctly register the LEFTSHIFT key state change, which is not documented or expected. Has anyone else experienced this issue, or is there a known workaround or fix?

Specifications of my machine

OS: Arch Linux x86_64 Host: XPS 17 9720 Kernel: 6.7.5-arch1-1 Uptime: 3 hours, 47 mins Packages: 1369 (pacman) Shell: zsh 5.9 Resolution: 1920x1200, 1920x1080 WM: dwm Theme: Raleigh [GTK2/3] Icons: Adwaita [GTK2/3] Terminal: tmux CPU: 12th Gen Intel i7-12700H (20) @ 4.600GHz GPU: Intel Alder Lake-P GT2 [Iris Xe Graphics] GPU: NVIDIA GeForce RTX 3050 Mobile Memory: 10185MiB / 15678MiB

[oOosys]

from evdev import UInput, ecodes as e
ui = UInput()
ui.write(e.EV_KEY, e.KEY_LEFTSHIFT, 1)	# <- press SHIFT to get "A" while pressing KEY_A
ui.write(e.EV_KEY, e.KEY_A, 1)			# <- KEY_A down/press
ui.write(e.EV_KEY, e.KEY_A, 0)			# <- KEY_A   up/release
ui.write(e.EV_KEY, e.KEY_LEFTSHIFT, 0)	# <- release SHIFT
ui.syn()
ui.close()

works correctly on my Linux Mint 21.3 Xfce machine running Python 3.10.

[mroavi]

True. I can't reproduce it anymore. Thanks.